includedir in suoders can be prefixed by "arroba"

[jordimassaguerpla]

Proposed Commit Message

Since version 1.9.1, @includedir can be used in the sudoers files
instead of #includedir:

https://github.com/sudo-project/sudo/releases/tag/SUDO_1_9_1

This happens to be the default in SUSE Linux enterprise sudoer package,
so cloudinit should take this into account.

Otherwise, cloudinit was adding an extra #includedir, which was
resulting on the files under /etc/sudoers.d being included twice, one by
@includedir from the SUSE package, one by the @includedir from
cloudinit. The consequence of this, was that if you were defining an
Cmnd_Alias inside any of those files, this was being defined twice and
creating an error when using sudo.

Signed-off-by: Jordi Massaguer Pla <jmassaguerpla@suse.de>```

## Additional Context
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## Test Steps
<!-- Please include any steps necessary to verify (and reproduce if
this is a bug fix) this change on a live deployed system,
including any necessary configuration files, user-data,
setup, and teardown. Scripts used may be attached directly to this PR. -->

Add "@includedir /etc/sudoers" into /etc/sudoers

Apply cloud-init and see you have both, "@includedir /etc/sudoers" and "#includedir /etc/sudoers".

After the fix, you should only have "@includedir"

## Checklist:
<!-- Go over all the following points, and put an `x` in all the boxes
that apply. -->
 - [ ] My code follows the process laid out in [the documentation](https://cloudinit.readthedocs.io/en/latest/topics/hacking.html)
 - [ ] I have updated or added any unit tests accordingly
 - [ ] I have updated or added any documentation accordingly

[jordimassaguerpla]

Hi. I already signed the individual contributors agreement. Could you retry the github action?

[jordimassaguerpla]

I was missing #787

[TheRealFalcon]

@jordimassaguerpla Thanks for this. The change looks good, but can you add a unit test or two as well? It would look similar and go in the same place as the one I'm linking here, but ensure that when a line containing the # or @ include is encountered, that it correctly keeps the line rather than appending a new one.
https://github.com/canonical/cloud-init/blob/master/tests/unittests/test_distros/test_generic.py#L109

[TheRealFalcon]

Thanks for the test. I left one small suggestion just to make sure we're hitting both include characters. After that, we should be good to merge!

[TheRealFalcon]

Looks good. Thanks!