Add support for `PermitRootLogin` option in the
`org.osbuild.sshd.config` stage.
I kept the "yes" and "no" values for consistency with other stage
options. While it will make the implementation in osbuild-composer
harder, it won't be impossible as we already have a precedence for doing
it this way (e.g. in the `org.osbuild.pam.limits.conf`).
Modify the stage unit tests to check the new option.
Remove the empty `org.osbuild.sshd.config` stage from `a.mpp.json`
since it does not add any value and it actually made the `tree-diff`
tool provide a weird tree diff results.
Fix#910
Signed-off-by: Tomas Hozza <thozza@redhat.com>