rhsm_repository: refactor handling of subscription-manager (#6783)

Create a small helper class Rhsm, so all the logic related to the
interaction with subscription-manager is grouped there:
- create the Rhsm object in main(), once the initial checks are done
- search subscription-manager as required (so there is no need to
  manually check it), and store its path for reuse
- store the common arguments for running subscription-manager
- move run_subscription_manager() to Rhsm as run_repos()
- get rid of the different list parameters: we list only all the
  repositories, so the other cases are not needed (and can be added
  easily, if needed)
- move get_repository_list() to Rhsm as list_repositories()

The execution of subscription-manager is improved as well:
- pass the arguments to run_command() directly as list, rather than
  joining the arguments to string, which run_command() will need to
  split again
- move the "repos" parameter directly in run_repos()
- explicitly disable the shell, already off by default
- disable the expansions of variables, as there are none

Adapt the unit test to the different way run_command() is called.

There should be no behaviour changes.

changelogs/fragments/6783-rhsm_repository-internal-refactor.yml

@@ -0,0 +1,6 @@
+minor_changes:
+  - |
+    rhsm_repository - the interaction with ``subscription-manager`` was
+    refactored by grouping things together, removing unused bits, and hardening
+    the way it is run; no behaviour change is expected
+    (https://github.com/ansible-collections/community.general/pull/6783).