NVP: Add LOG.exception to see why router was not created

Add missing LOG.exception so that it exposes what error occurred.

Change-Id: I9194795678a6be59eb1d2555dfa99ca7a035c418
Closes-bug: #1269152
Base ML2 bulk support on the loaded drivers

Changes the ML2 plugin bulk support flag to
be based on the bulk support of the underlying
drivers.

Closes-Bug: #1272490
Change-Id: I28281c9ecc1696b929c7e0125d02a37946948744
Merge "Base ML2 bulk support on the loaded drivers"
Merge "NVP: Add LOG.exception to see why router was not created"