NSS: Print FQDN for groups with mixed domain membership
    
    This patch is a workaround until
    https://fedorahosted.org/sssd/ticket/2129 is fixed properly.
    
    Consider a group entry such as:
     cn: subgroup@subdom
     ghost: someuser
     ghost: anotheruser@subdom
    
    Currently in order to print all group members as FQDN (which is the default
    for AD provider), the code needs to iterate over the ghost attributes and
    parse them into (name,domain) and optionally re-add the domain.
    
    The proper fix would be to store always just the FQDN in the hardcoded
    form of user@domain