Designer should clean up preview vdbs every time Teiid Server connection is made.

It is quite possible that that the preview vdbs deployed into a Teiid Server get out of sync with the Designer when the Server is restarted independent of the Designer. Since Designer keeps track of its preview vdbs based on workspace uuid, it is better that it cleans up its preview vdbs and data sources when the connection is made and connection is cleaned up. These actions may happen at both startup and shutdown of the Designer and also at the creation and discard of the Server settings once the Designer started. Otherwise there may be orphan VDBs left over in the Server.