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

plugins/modules/redhat_subscription.py

@@ -26,6 +26,11 @@ notes:
       config file and default to None.
     - It is possible to interact with C(subscription-manager) only as root,
       so root permissions are required to successfully run this module.
+    - Since community.general 6.5.0, credentials (that is, I(username) and I(password),
+      I(activationkey), or I(token)) are needed only in case the the system is not registered,
+      or I(force_register) is specified; this makes it possible to use the module to tweak an
+      already registered system, for example attaching pools to it (using I(pool), or I(pool_ids)),
+      and modifying the C(syspurpose) attributes (using I(syspurpose)).
 requirements:
     - subscription-manager
     - Optionally the C(dbus) Python library; this is usually included in the OS
@@ -1073,7 +1078,7 @@ def main():
                             ['activationkey', 'environment'],
                             ['activationkey', 'auto_attach'],
                             ['pool', 'pool_ids']],
-        required_if=[['state', 'present', ['username', 'activationkey', 'token'], True]],
+        required_if=[['force_register', True, ['username', 'activationkey', 'token'], True]],
     )
 
     if getuid() != 0:
@@ -1155,6 +1160,8 @@ def main():
                 else:
                     module.exit_json(changed=False, msg="System already registered.")
         else:
+            if not username and not activationkey and not token:
+                module.fail_json(msg="state is present but any of the following are missing: username, activationkey, token")
             try:
                 rhsm.enable()
                 rhsm.configure(**module.params)