webadmin: Add ISO domain option in data center "Guide me" is missing

A bug in the guide me model caused the add ISO domain option in the data
center guide me to be only visible in case there is no ISO domain in the
whole setup, this is wrong since the user should be able to add a new ISO
to a data center regardless to whether another ISO already exists in the
system

Change-Id: I76c61f65e6cf7dbdaf631e2c9b7663f9dfffd16a
Bug-Url: https://bugzilla.redhat.com/1070152
Signed-off-by: Tal Nisan <tnisan@redhat.com>