This will allow one to define what SELinux context a given user gets
on a given machine. A rule can contain a set of users and hosts or it
can point to an existing HBAC rule that defines them.
https://fedorahosted.org/freeipa/ticket/755
This construct allows to have a group of ipaExternalMember attributes, that can
be nested in a normal ipa Group ('memberOf' is allowed).
It cannot contain normal ipa users/groups and cannot be nested with another
group of the same type ('member' is not allowed).