redhat_subscription: require credentials only when needed (#5664)

The module currently has a static 'required_if' statement for its
parameters that forces any of 'username' or 'activationkey' or 'token'
in case state=present; while this is generally a good idea, it can be
an extra requirements in some cases. In particular, if the system is
already registered, there is no need for credentials -- some of the
operations of the module, such as manipulating pools, can be done
perfectly without credentials.

Hence:
- change the static 'required_if' to require credentials only when
  forcing the registration
- check for credentials manually when a registration is needed, i.e.
  on an unregistered system; the fail message is the same as the one
  shown by 'required_if'

Adapt the tests to this new situation:
- test_without_required_parameters now needs to mock an unregistered
  system
- add a new version of test_without_required_parameters to test an
  already registered system
- add a simple test case for only state=present usable on an already
  registered system
- remove the credentials from a test case for pool attachment that
  mocks an already registered system

changelogs/fragments/5664-redhat_subscription-credentials-when-needed.yaml

@@ -0,0 +1,4 @@
+minor_changes:
+  - redhat_subscription - credentials (``username``, ``activationkey``, and so on) are required now
+    only if a system needs to be registered, or ``force_register`` is specified
+    (https://github.com/ansible-collections/community.general/pull/5664).