Unset the NFSv4 domain in idmapd.conf when it's unset in the TUI

Previously, removing the NFSv4 domain did not properly clear the
configuration in idmapd.conf. Check that it's not an empty string,
and comment out "Domain", as well as setting it to a null value so
we can reset it again.

Change-Id: Ie689b27fe8808ffed2b50bde347e03a970aba2c8
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=960833
Signed-off-by: Ryan Barry <rbarry@redhat.com>