rhsm_release: improve the execution of subscription-manager (#6669)

- pass the arguments to run_command() directly as list, rather than
  joining the arguments to string, which run_command() will need to
  split again
- disable the expansions of variables, as there are none

Adapt the unit test to the different way run_command() is called,
factorizing the kwargs for run_command() so there is less repetition.

There should be no behaviour changes.

changelogs/fragments/6669-rhsm_release-internal-sub-man-exec.yml

@@ -0,0 +1,5 @@
+minor_changes:
+  - |
+    rhsm_release - improve/harden the way ``subscription-manager`` is run;
+    no behaviour change is expected
+    (https://github.com/ansible-collections/community.general/pull/6669).