allow empty groups
    https://bugzilla.redhat.com/show_bug.cgi?id=526141
    Resolves: bug 526141
    Bug Description: allow empty groups
    Reviewed by: nhosoi (Thanks!)
    Fix Description: Change groupOfNames and groupOfUniqueNames to allow empty
    groups by changing the member/uniqueMember attribute from MUST to MAY.
    Platforms tested: RHEL5 x86_64
    Flag Day: no
    Doc impact: no