Add support for adding external members to ipagroup.

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