Decompose DVR CSNAT L3 Agent from Compute Node L3 Agent

Decompose DVR CSNAT L3 Agent from Compute Node L3 Agent

Currently the same dvr router class is used both by the L3 Agent
in the compute nodes that is responsible for the virtual routers
namespace and the fip namespace and also used by the centralized
SNAT L3 Agent in the network node.
This is the first step to decompose the two into different
classes.

The above means that we have one class of DVR router which is used
for two jobs (the virtual router namespace wiring and the fips wiring
in the compute node in one hand and the centralized snat wiring in the other)
The end goal of this patch is to separate the two into different classes
which will also help maintaining it and also help projects that want
to use one but not the other (for example only use the centralized
SNAT behaviour with there own DVR implementation)

Change-Id: I581a097b9e7c49f20d0eb0e4ca66a25e90d9511b
Partial-Bug: #1458541
Partially-Implements: blueprint dvr-router-code-decompose