Move _set_subnet_arp_info to dvr_router

Move _set_subnet_arp_info to dvr_router

Further teasing DVR apart from the agent and other router types.  This
change illustrates the need to use plugin_rpc and context from the
agent.  It leaves this as a dependence of dvr_router on the instance
of the agent.  So, when the agent creates a dvr_router, it passes
itself in as an argument.  It doesn't do this with other router types.

I would like to eliminate the dependence of RPC while processing a
router.  However, this is just a refactor and at least decouples it as
much as possible and isolates the dependence to DVR only.

Change-Id: Icb2154648b7aee6ddf781c48f1801ff97d35c6e6
Partially-Implements: bp/restructure-l3-agent