When adding a duplicate member to a group, an error message is issued,
informing the user that the entry is already a member of the group.
Similarly, when trying to delete an entry which is not a member,
an error message is issued, informing the user that the entry is not
a member of the group. These error messages were missing in case of
external members.
This patch also adds support for using the AD\name or name@ad.domain.com
format in ipa group-remove-member command. This format was supported in
group-add-member, but not in group-remove-member.
Unit test file covering these cases was also added.
https://fedorahosted.org/freeipa/ticket/3254
