This patch add support for adding external members to ipagroup which
have the `external` attribute set. It adds another attribute to the
module, `external_members`, which is a list of users or groups from
an external trust, to be added to the group.
This patch requires server-trust-ad to be tested, as such, the tests
have been guarded by a test block, for when such tests are available
in ansible-freeipa CI.
Fixes issue #418
Some attributes are not present in all supported versions of FreeIPA,
and this might cause tests to fail due to unsupported versions.
This patch add the means to test if a test can be executed based on
the target host FreeIPA version.
