Adds API for mergeVdbs in IExecutionAdmin

* Restores the merge method to Teiid 7 ExecutionAdmin, only allowing the
  import vdb functionality to 8+ servers.

* Merge should remain deprecated since it is only for backward compatibility

SHOULD BE MERGED ALONG WITH:
https://github.com/Teiid-Designer/teiid/pull/1
https://github.com/Teiid-Designer/teiid/pull/2