Fix ipa.service unit re. gssproxy
    
    ipa.service unit defines Requires=gssproxy. Because of this, during
    ipa-server-upgrade, the restart of gssproxy triggers a restart of ipa unit
    (hence stopping LDAP server and breaking the connection api.Backend.ldap2).
    Calls using this connection after gssproxy restart fail and ipa-server-upgrade
    exits on failure.
    The fix defines Wants=gssproxy to avoid the restart of ipa.service
    
    https://fedorahosted.org/freeipa/ticket/6705
    
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>