Expunge session contents between plugin requests

In the NVP plugin, metadata processing performs several plugin operations
with the same context (and db session). The first operation might leave
persisted objects in the session instance which then conflict with objects
created in the second operation.

Closes-Bug: #1266482

Change-Id: Ic9e2b952013610647aa222f1a8b6e33504b54b37
Merge "Expunge session contents between plugin requests"