ipa-client-automount: fix '--idmap-domain DNS' logic
    
    Previously '--idmap-domain DNS' would assume the Domain
    parameter of idmapd.conf was already absent. With this
    fix, the Domain parameter is always removed and the
    configuration file is always backuped.
    
    Related-to: https://pagure.io/freeipa/issue/7918
    Fixes: https://pagure.io/freeipa/issue/7988
    Signed-off-by: François Cami fcami@redhat.com
    Reviewed-By: Christian Heimes <cheimes@redhat.com>