core: Set Sysprep domain to editable field

Currently we have a close list of domains (which is set via
engine-manage-domains tool)

Change-Id: I0370e63593035e80851852dca40a480fdccf46fa
Bug-Url: https://bugzilla.redhat.com/1072414
Signed-off-by: Shahar Havivi <shaharh@redhat.com>