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: 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

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'] }}"

__$rolename_is_redhat_distro: "{{ ansible_distribution in __$rolename_rh_distros }}"
__$rolename_is_redhat_distro_fedora: "{{ ansible_distribution in __$rolename_rh_distros_fedora }}"

Then the conditionals can be rewritten as:

some_var: "{{ 'some value' if __$rolename_is_redhat_distro else 'other value' }}"
another_var: "{{ 'some value' if __$rolename_is_redhat_distro_fedora else 'other value' }}"

...

- name: Do something
  when: __$rolename_is_redhat_distro | bool
  ...
- name: Do something else
  when: __$rolename_is_redhat_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/redhat_clone_vars.yml:

vars_files:
  - vars/redhat_clone_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.

Signed-off-by: Rich Megginson rmeggins@redhat.com