webadmin: Disable + sign in custom properties

When there are no more possible values disable the + sign
in custom properties

Change-Id: I40e391566dd3bcced2a345ce6a56e2244ac94294
Bug-Url: https://bugzilla.redhat.com/1001159
Signed-off-by: Noam Slomianko <nslomian@redhat.com>
Signed-off-by: Gilad Chaplik <gchaplik@redhat.com>