Add an option for rhel to force the use of hostname config

[hamalq]

add prefer_fqdn_over_hostname config option

the above option allow the user to control the
behavior of a distro hostname selection
so if the user send hostname, FQDN Configs
if prefer_fqdn_over_hostname is true the
The FQDN will be selected as hostname
if false the hostname will be selected

LP: #1921004

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• My code follows the process laid out in the documentation
• I have updated or added any unit tests accordingly
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[TheRealFalcon]

Hi @hamalq , I think if we had a distro-independent setting along the lines of default_hostname: <hostname|fqdn>, that'd be more accessible to everybody.

@OddBloke , do you have strong opinions here? I'm hesitant about adding a new top-level key, but given how things currently work, I'm not really sure there's a better way to allow this.

[OddBloke]

I strongly agree that we should name it independent of distro. We have been trying to avoid introducing new top-level keys, as you say: given that we already use the hostname key, there isn't an obvious name for a key that could gather all of these options together.

default_hostname isn't quite right either: the value of the setting isn't a hostname, it's a type of hostname. And, I suppose, it isn't really a default either: it's defining what the defaulting behaviour should be. Perhaps preferred_hostname_type?

Stepping back, actually, might it be easier to express this as a more explicit boolean: prefer_fqdn_over_hostname, for example? We already know all the types of hostname that we need to deal with, and there are two of them. RHEL should default to true for it, everything else should default to false.

[blackboxsw]

Thank you for the branch @hamalq in order to land this I think we need to minmally extend unittests which cover these affected behaviors in

https://github.com/canonical/cloud-init/blob/master/tests/unittests/test_handler/test_handler_set_hostname.py

Also it would be nice to extend the integration test below with a sample #coud-config which provides this new key
tests/integration_tests/modules/test_set_hostname.py

[blackboxsw]

I think what remains is documentation of this feature. I'm testing locally now to confirm behavior and double checking the need for distro.set_option

[blackboxsw]

I confirm that this approach is the best we have at the moment. distro class has no visibility to the merged sys_cfg which contains user-data cloud-config settings (which the datasource instances have). I was under the impression we might be able to get to it thourgh distro._cfg but that's a different subset of config entirely.

confirmed that hostnames are properly set on this new version cloud-init.

[TheRealFalcon]

Looks good to me! I took the liberty of pushing a couple small changes:

1. The same doc update you made to cc_set_hostname.py was also needed in cc_update_hostname.py
2. Added an integration test for when prefer_fqdn_over_hostname is false
3. Updated some unit tests to include both hostname and fqdn to make sure we're always choosing the right one
4. Also updated some unit tests to not skip their verification if we're on a systemd system.

Assuming CI passes, I should be able to get this merged later today.