Improve handling of GIDs when migrating groups
    
    Since IPA v2 server already contain predefined groups that may collide
    with groups in migrated (IPA v1) server (for example admins, ipausers),
    users having colliding group as their primary group may happen to belong
    to an unknown group on new IPA v2 server.
    
    Implement --group-overwrite-gid option to overwrite GID of already
    existing groups to prevent this issue.
    
    https://fedorahosted.org/freeipa/ticket/1866