Change default netpartition behavior in nuage plugin

Allows the nuage plugin handle the case where default netpartition
already exists on the VSD (back-end controller)

Change-Id: Ia5785c1c313cf30b8bad6cda1d0e9819784e06d8
Closes-Bug: #1319892
Consistently use jsonutils instead of specific implementation

jsonutils have several benefits in comparison to pure json
implementation, like enabling C boosted encoders and decoders for
Python2.6 by using simplejson when available.

Change-Id: I24d0cd442e8d9d89fac50e43fc97f7bb4a293c3d
Closes-Bug: 1329496
Move _filter_non_model_columns method to CommonDbMixin

Such way other non-core plugins may avoid duplicating this small
piece of code.

Change-Id: I00ae0b590fc2ad46d3667d18f29fccb5b40bde82
Merge "Change default netpartition behavior in nuage plugin"
Merge "Consistently use jsonutils instead of specific implementation"
Merge "Move _filter_non_model_columns method to CommonDbMixin"