Raise error when attaching item without dialog to a catalog

I've added a validation function into the ServiceTemplate model, it fixes the bug when editing a catalog item.
The obsolete disabled order button for dialog-less items was removed.

https://bugzilla.redhat.com/show_bug.cgi?id=1145266
Merge pull request #759 from skateman/service-catalog-dialog-fix

Raise error when attaching item without dialog to a catalog