New utils method that returns the lists of members.

[dkarpele]

Extend netgroup and sudorule modules to support external
users and hosts wherever possible.
Add tests for ipanetgroup and ipasudorule.

Problem statement:

    - name: Ensure sudorule is present with users and hosts (action member)
      ipasudorule:
        name: testrule2
        user:
          - external-user
        action: member
    - name: Ensure sudorule is present with users and hosts (action member) again
      ipasudorule:
        name: testrule2
        user:
          - external-user
        action: member

After execution of the first task with external users ansible returns changed as expected, after second it still returns changed - it's a bug. This PR fixes it. After the second task ansible will return ok.
"External" entities are:
for ipasudorule: externalhost, externaluser, ipasudorunasextuser, ipasudorunasextgroup
for ipanetgroup: externalhost

[t-woerner]

Please add information about externalhost and externaluser to the commit message and also as a comment to the code to make is easier to understand where these are coming from and what is returned in there.
Please also change the name of the function to be self-explaining. Something like concat_attr_list or similar to show that is concatenating lists.

[rjeffman]

Please, add the new test playbooks to the "*_client_context" test playbook.

Client context test is important for any method dealing with lists as lists are returned as tuples in this case.

[rjeffman]

About the client context tests, I'd like to have a single "test_*_client_context.yml" file per module, so you can simply add the regular test file as "import_playbook" to the existing client context test playbook.

As this might be debatable, @t-woerner, do you agree?

[dkarpele]

About the client context tests, I'd like to have a single "test_*_client_context.yml" file per module, so you can simply add the regular test file as "import_playbook" to the existing client context test playbook.

As this might be debatable, @t-woerner, do you agree?

Thanks @rjeffman, it makes sense as for example there are already test_netgroup_member.yml and test_netgroup.yml tests in test_netgroup_client_context.yml. I update the PR.

[rjeffman]

LGTM.

I really like the idiom allowed by concat_list_attr. Thank you for this!