TEIIDDES-1604: Rebuild preview vdbs on default server change

* Since preview vdbs can contain metadata incompatible between server
  versions, a change in the default server version must require all preview
  vdbs to be delete and rebuilt against the new default server

* Preview manager should not be disconnecting the teiid server. As a
  background service this can cause confusion for the user
Merge pull request #115 from phantomjinx/master

Preview VDB Cleanup on server version change