Improve Nicira plugin test coverage

Blueprint nvp-test-coverage

This patch improves the Nicira plugin test coverage by:
- adding a suite of test cases for nvplib
- removing unused exception handling code
- removing 'vestigial' code in nvp plugin modules

This patches also fixes a few minor glitches with the plugin
and the fake api client.

Change-Id: Ia319f556a815e5061b867a97d834123786218fec
Merge "Improve Nicira plugin test coverage"