refactor: Use vars/RedHat_N.yml symlink for CentOS, Rocky, Alma wherever possible

[richm]

We have a lot of requests to support Rocky and Alma in various system roles. The
first part of adding support is adding vars/ files for these platforms. In
almost every case, for a given major version N, the vars file RedHat_N.yml can
be used for CentOS, Rocky, and Alma. Rather than making a copy of the
RedHat_N.yml file, just use a symlink to reduce size and maintenance burden, and
standardize this across all system roles for consistency.

NOTE: There is no Alma or Rocky version 7 or less.

NOTE: OracleLinux is not a strict clone, so we are not going to do this for
OracleLinux at this time. Support for OracleLinux will need to be done in
separate PRs. For more information, see
linux-system-roles/cockpit#130

Question: Why not just use ansible_facts["os_family"] == "RedHat"?

Answer: This is what Ansible uses as the RedHat os_family:
https://github.com/ansible/ansible/blob/1e6ffc1d02559a26def6c9c3b07baf27032865a2/lib/ansible/module_utils/facts/system/distribution.py#L511
There are a lot of distributions in there. I know that Fedora is not a clone of
RHEL, but it is very closely related. Most of the others are not clones, and it
would generally not work to replace ansible_distribution in ['CentOS', 'Fedora',
'RedHat'] with ansible_facts['os_family'] == 'RedHat' (but it would probably
work in specific cases with specific distributions). For example, OracleLinux
is in there, and we know that doesn't generally work. The only ones we can be
pretty sure about are RedHat, CentOS, Fedora, AlmaLinux, and Rocky.

Question: Does my role really need this because it should already work on
RHEL clones?

Answer: Maybe not - but:

• it doesn't hurt anything
• it's there if we need it in the future
• the role will be inconsistent with the other system roles if we don't have this

Question: Why do I need the tests/vars/rh_distros_vars.yml file? Doesn't
the test load the vars from the role?

Answer: No, the test does not load the vars from the role until the role is
included, and many tests use version and distribution before including the role.

Question: Do we need to change the code now to use the new variables?

Answer: No, not now, in subsequent PRs, hopefully by Alma and Rocky users.

Note that there may be more work to be done to the role to fully support Rocky
and Alma. Many roles have conditionals like this:

some_var: "{{ 'some value' if ansible_distribution in ['CentOS', 'RedHat'] else 'other value' }}"
another_var: "{{ 'some value' if ansible_distribution in ['CentOS', 'Fedora', 'RedHat'] else 'other value' }}"

...

- name: Do something
  when: ansible_distribution in ['CentOS', 'RedHat']
  ...
- name: Do something else
  when: ansible_distribution in ['CentOS', 'Fedora', 'RedHat']
  ...

Adding Rocky and AlmaLinux to these conditionals will have to be done
separately. In order to simplify the task, some new variables are being
introduced:

__$rolename_rh_distros:
  - AlmaLinux
  - CentOS
  - RedHat
  - Rocky

__$rolename_rh_distros_fedora: "{{ __$rolename_rh_distros + ['Fedora'] }}"

__$rolename_is_rh_distro: "{{ ansible_distribution in __$rolename_rh_distros }}"
__$rolename_is_rh_distro_fedora: "{{ ansible_distribution in __$rolename_rh_distros_fedora }}"

Then the conditionals can be rewritten as:

some_var: "{{ 'some value' if __$rolename_is_rh_distro else 'other value' }}"
another_var: "{{ 'some value' if __$rolename_is_rh_distro_fedora else 'other value' }}"

...

- name: Do something
  when: __$rolename_is_rh_distro | bool
  ...
- name: Do something else
  when: __$rolename_is_rh_distro_fedora | bool
  ...

For tests - tests that use such conditionals will need to use vars_files or
include_vars to load the variables that are defined in
tests/vars/rh_distros_vars.yml:

vars_files:
  - vars/rh_distros_vars.yml

We don't currently have CI testing for Rocky or Alma, so someone wanting to run
tests on those platforms would need to change the test code to use these.