[1LP][RFR] De-duplicate code for VM / VMCollection retirement

[tpapaioa]

This is a follow-up to #9540, making some additional changes that were suggested in review.

1.) cleanup of code that loops over vm instances, e.g.,

-    for num in range(2):
-        vms[num].cleanup_on_provider()
+    for vm in vms:
+        vm.cleanup_on_provider()

2.) De-duplication of the common code shared by the set_retirement_date methods in the VM and VMCollection classes. The parameter validation and form filling is the same in both classes, and the only differences are in:

• The navigation to the Set/Remove Retirement Date view
• The view instantiation / validation after saving or cancelling any retirement date changes.

I've created a RetirementMixin class and moved the set_retirement_date method there. The code that needs to be overridden at the VM or VMCollection level is moved to the new navigate_to_set_retirement and post_set_retirement methods.

{{ pytest: cfme/tests/cloud_infra_common/test_retirement.py -v --long-running --use-provider vsphere67-nested --use-provider ec2west -k 'test_retirement_now or test_set_retirement or test_unset_retirement_date or test_resume_retired_instance' }}

[john-dupuy]

Some small comment, but overall LGTM, awesome work @tpapaioa

[tpapaioa]

One PRT test in the latest commit failed with a TimedOutError exception, but it's unrelated to this PR, so I'm changing the status back to RFR.

[mshriver]

Two small things to discuss, otherwise these changes look great, thanks @tpapaioa !

[dajoRH]

I detected some fixture changes in commit cc2e271

The local fixture retire_vm_pair is used in the following files:

• cfme/tests/cloud_infra_common/test_retirement.py
  • test_retirement_now_multiple
  • test_set_retirement_date_multiple
  • test_set_retirement_offset_multiple

Please, consider creating a PRT run to make sure your fixture changes do not break existing usage 😃