[Options] Add test of skip_plugins before referring it

[pmoravec]

When a component honours plugopts but not skip_plugins Option, dont update plugopts per skip_plugins.

This is rather theoretical scenario, though.

Resolves: #3904

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[packit-as-a-service]

Congratulations! One of the builds has completed. 🍾

You can install the built RPMs by following these steps:

• sudo yum install -y dnf-plugins-core on RHEL 8
• sudo dnf install -y dnf-plugins-core on Fedora
• dnf copr enable packit/sosreport-sos-3904
• And now you can install the packages.

Please note that the RPMs should be used only in a testing environment.

[pmoravec]

Bit artificial reproducer: On one version of sos upload PR, there was + 'plugopts': [], added to arg_defaults, but no skip_plugins there. That was ridiculous for upload component, but let assume there is such a component.

Then having non-empty [plugin_options] in sos.conf, I was getting weird exception:

# python3 bin/sos upload fooo
Could not initialize 'upload': 'SoSOptions' object has no attribute 'skip_plugins'
Traceback (most recent call last):
  File "/root/sos-main/bin/sos", line 21, in <module>
    sos = SoS(sys.argv[1:])
  File "/root/sos-main/sos/__init__.py", line 118, in __init__
    self._init_component()
  File "/root/sos-main/sos/__init__.py", line 183, in _init_component
    raise err
  File "/root/sos-main/sos/__init__.py", line 178, in _init_component
    self._component = _to_load(self.parser, self.args, self.cmdline)
  File "/root/sos-main/sos/upload/__init__.py", line 58, in __init__
    super().__init__(parser, args, cmdline)
  File "/root/sos-main/sos/component.py", line 103, in __init__
    self.opts = self.load_options()  # lgtm [py/init-calls-subclass]
  File "/root/sos-main/sos/component.py", line 262, in load_options
    opts.update_from_conf(self.args.config_file, self.args.component)
  File "/root/sos-main/sos/options.py", line 232, in update_from_conf
    if not key.split('.')[0] in self.skip_plugins:
AttributeError: 'SoSOptions' object has no attribute 'skip_plugins'
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