Added a sudo method to cli.Client to avoid boilerplate.

As part of our `less boilerplate` effort I added a new method `sudo`
to `cli.Client`.

In a lot of test cases there are code like:

```python
sudo = () if cli.is_root(cfg) else ('sudo',)
...
client.run(sudo + ('command_name', 'argument1', ...)
```

The idea is to replace the above boilerplate with

```python
client.sudo('command_name', 'argument1', ...)
```

The method name is explicly sayig what it does as `sudo` stands for `SUperuser` + `DO` and it follows the same naming seeing on other projects like Fabric and Ansible (internak modules implementation).

With this change we will be able to remove the `sudo = () if cli.is_root(cfg) else ('sudo',)` boilerplate in those Pulp-2-tests referenced:

```bash

pulp_2_tests/tests/docker/utils.py:    sudo = '' if cli.is_root(cfg) else 'sudo'
pulp_2_tests/tests/puppet/api_v2/test_install_distributor.py:        sudo = () if cli.is_root(self.cfg) else ('sudo',)
pulp_2_tests/tests/rpm/api_v2/test_content_sources.py:        sudo = () if cli.is_root(cls.cfg) else ('sudo',)
pulp_2_tests/tests/rpm/api_v2/test_content_sources.py:        sudo = () if cli.is_root(cls.cfg) else ('sudo',)
pulp_2_tests/tests/rpm/api_v2/test_content_sources.py:    sudo = '' if cli.is_root(cfg) else 'sudo'
pulp_2_tests/tests/rpm/api_v2/test_crud.py:        sudo = () if cli.is_root(self.cfg) else ('sudo',)
pulp_2_tests/tests/rpm/api_v2/test_download_policies.py:        sudo = '' if cli.is_root(cls.cfg) else 'sudo '
pulp_2_tests/tests/rpm/api_v2/test_errata.py:        # sudo = () if cli.is_root(cfg) else ('sudo',)
pulp_2_tests/tests/rpm/api_v2/test_errata.py:        sudo = () if cli.is_root(cfg) else ('sudo',)
pulp_2_tests/tests/rpm/api_v2/test_export.py:        self.__sudo = None
pulp_2_tests/tests/rpm/api_v2/test_export.py:            self.__sudo = '' if cli.is_root(self.cfg) else 'sudo '
pulp_2_tests/tests/rpm/api_v2/test_iso_sync_publish.py:        sudo = () if cli.is_root(self.cfg) else ('sudo',)
pulp_2_tests/tests/rpm/api_v2/test_modularity.py:        sudo = () if cli.is_root(cfg) else ('sudo',)
pulp_2_tests/tests/rpm/api_v2/test_republish.py:        sudo = () if cli.is_root(self.cfg) else ('sudo',)
pulp_2_tests/tests/rpm/api_v2/test_rich_weak_dependencies.py:        sudo = () if cli.is_root(cfg) else ('sudo',)
pulp_2_tests/tests/rpm/api_v2/test_rsync_distributor.py:        sudo = () if cli.is_root(cfg) else ('sudo',)
pulp_2_tests/tests/rpm/api_v2/test_rsync_distributor.py:        sudo = () if cli.is_root(cfg) else ('sudo',)
pulp_2_tests/tests/rpm/api_v2/test_rsync_distributor.py:        sudo = '' if cli.is_root(cfg) else 'sudo '
pulp_2_tests/tests/rpm/api_v2/test_service_resiliency.py:        sudo = '' if cli.is_root(self.cfg) else 'sudo'
pulp_2_tests/tests/rpm/api_v2/test_service_resiliency.py:        sudo = () if cli.is_root(self.cfg) else ('sudo',)
pulp_2_tests/tests/rpm/api_v2/utils.py:        sudo = '' if cli.is_root(cfg) else 'sudo '
pulp_2_tests/tests/rpm/api_v2/utils.py:        sudo = '' if cli.is_root(cfg) else 'sudo '
pulp_2_tests/tests/rpm/api_v2/utils.py:        sudo = () if cli.is_root(cfg) else ('sudo',)
pulp_2_tests/tests/rpm/api_v2/utils.py:        sudo = '' if cli.is_root(cfg) else 'sudo '
pulp_2_tests/tests/rpm/api_v2/utils.py:    sudo = () if cli.is_root(cfg) else ('sudo',)
pulp_2_tests/tests/rpm/cli/test_copy_units.py:        sudo = () if cli.is_root(cfg) else ('sudo',)
pulp_2_tests/tests/rpm/cli/test_process_recycling.py:    sudo = () if cli.is_root(cfg) else ('sudo',)
pulp_2_tests/tests/rpm/cli/test_process_recycling.py:        sudo = () if cli.is_root(cfg) else ('sudo',)

```

pulp_smash/cli.py

@@ -213,6 +213,8 @@ def __init__(self, cfg, response_handler=None, pulp_host=None):
         else:
             self.response_handler = response_handler
 
+        self.cfg = cfg
+
     def run(self, args, **kwargs):
         """Run a command and ``return self.response_handler(result)``.
 
@@ -234,6 +236,18 @@ def run(self, args, **kwargs):
         completed_process = CompletedProcess(args, code, stdout, stderr)
         return self.response_handler(completed_process)
 
+    def sudo(self, args, **kwargs):
+        """Run a command as superuser.
+
+        This method is a wrapper around :meth:`pulp_smash.cli.Client.run`
+        which prior to execution checks if user is root and if not
+        adds ``sudo`` as the first argument of the command.
+        """
+        if args[0] != 'sudo' and not is_root(self.cfg):
+            args = list(args)
+            args.insert(0, 'sudo')
+        return self.run(tuple(args), **kwargs)
+
 
 class BaseServiceManager(metaclass=ABCMeta):
     """A base service manager.

tests/test_cli.py

@@ -167,6 +167,19 @@ def test_explicit_pulp_host(self):
             plumbum.machines.SshMachine.assert_called_once_with(
                 cfg.hosts[1].hostname)
 
+    def test_sudo(self):
+        """Test run commands as sudo."""
+        cfg = _get_pulp_smash_config(hosts=[
+            config.PulpHost(
+                hostname=socket.getfqdn(),
+                roles={'shell': {}},
+            )
+        ])
+        client = cli.Client(cfg)
+        with mock.patch.object(client, 'run') as run:
+            client.sudo(('ls',))
+            run.assert_called_once_with(('sudo', 'ls'))
+
 
 class IsRootTestCase(unittest.TestCase):
     """Tests for :class:`pulp_smash.cli.is_root`."""