This allows bootstrapping the user on a base image, by using
default ssh settings during provisioning and user defined
ssh settings when doing vagrant ssh.

TESTING INSTRUCTIONS:

1. Create user_playbooks/ssh-test.yml to bootstrap the user (edit the usernames to match those you will test):

---
- hosts: all
  become: true
  vars:
    unprivileged_user_username: wclark
    unprivileged_user_import_ssh_pub_key_github: wbclark

    - name: "Include unprivileged_user role"
      ansible.builtin.include_role:
        name: unprivileged_user

NOTE: As written, this requires the feature to add the user's ssh public key to authorized_keys directly from github via their github username, from #1570

2. Add the following within vagrant/boxes.d:

fedora34-ssh-test:
  box: fedora34
  ssh_forward_agent: true
  ssh_username: wclark
  ssh_keys_only: false
  use_ssh_settings_vagrant_ssh_only: true
  ansible:
    playbook:
      - 'user_playbooks/ssh-test.yml'

3. Make sure ssh-agent is started and your private key has been added to it

4. Run vagrant up fedora34-ssh-test and observe this still uses the default vagrant user to run provisioning tasks, including creating the new user and managing its authorized_keys.

5. Run vagrant ssh-config fedora34-ssh-test and observe the ssh_username from the box definition defined here, even though it was not used in step 4.

6. Run vagrant ssh fedora34-ssh-test and observe that it authenticates as the ssh_username with credentials from the ssh-agent.

7. As the ssh_username user, logged in on fedora34-ssh-test box, test that the ssh-agent was forwarded to the box by using the stored credentials to authenticate to github: ssh -T [email protected]

EXPECTED RESULTS: vagrant up uses the default vagrant user instead of ssh_username, while vagrant ssh-config and vagrant ssh use ssh_username. all above steps are successful

ACTUAL RESULTS (prior to this commit): vagrant up fails as it tries to use the ssh_username which doesn't exist until after provisioning has been run. It will appear to hang on waiting for ssh to become available while adding VAGRANT_LOG=debug will show it attempting the connection with the not-yet-existing user.