TEIIDDES-1833: Clean vdbs on connection of client to teiid instance

* PreviewManager
 * On connection of the client to the teiid instance, clean the latter of
   all preview vdbs.
 * This means whenever a server is started (and the instance successfully
   connected to) or when the refresh button is clicked on, the manager will
   find all preview vdbs and remove them accordingly.

* ExecutionConfigurationEvent
 * Requires an additional event for the specific case of the instance being
   connected to. Refresh occurs too often to be of use.

* TeiidServer
 * Fire the connection event only when the instance has been successfully
   connected.