caacl: fix regression in rule instantiation
The Principal refactor causes service collections
('memberservice_service' attribute) to return Principal objects
where previously it returned strings, but the HBAC machinery used
for CA ACL enforcement only handles strings. Update the code to
stringify service Principal objects when adding them to HBAC rules.
Fixes: https://fedorahosted.org/freeipa/ticket/6146
Reviewed-By: Martin Basti <mbasti@redhat.com>