ipatests: add tests for the new NFSv4 domain option of ipa-client-automount
    
    This commit tests the--idmap-domain knob with the following behavior:
    - if not present, default to IDM domain (current behavior)
    - if equal to DNS (magic value), set nothing and let idmapd autodetect domain
    - otherwise set Domain in idmap.conf to the value passed by this parameter
    
    Related to: https://pagure.io/freeipa/issue/7918
    Signed-off-by: François Cami <fcami@redhat.com>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>