Limiting import functionality for teiid 7

* Restores the merge functionality to 7 servers, only allowing the import
  vdb functionality to 8+ servers.

Not appropriate to restore a mergeVdb method to IExecutionAdmin since the original method does a lot of similar logic to the current code so an if/else tweak is more appropriate, especially given that ExecutionAdmin implementations have no business creating a teiid designer Vdb object.