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
linuxbridge-agent: process port updates in the main loop

This patch changes the way the linuxbridge agent processes port update
notifications. It does the same for the lb-agent as was done for the
ovs-agent in I219c6bdf63b0b5e945b655677f9e28fa591f03cd.
Now the RPC call just adds the updated tap device name to a set of
updated devices, and the actual processing is done in the main RPC loop.
This should solve the problems were port_update RPCs were competing
with the main loop/greenthread and with each other to do the same task
- like adding a tap or interface to a bridge - which lead to races and
was inefficient. Now repeated port_update notifications received within
a single main loop iteration will be coalesced and processed only once.

Closes-Bug: 1256950
Change-Id: I7fd48542f12b39ffc1346d1a6c9a387ecda6d812
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 "linuxbridge-agent: process port updates in the main loop"
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"