manual add/remove router for dvr_snat agent

manual add/remove router for dvr_snat agent

This patch is to address the failure of manual move of
dvr_snat routers from one service node to another.

The entry in the csnat_l3_agent_bindings table is now removed
during the router to agent unbind operation.
Appropriate notification is now sent to the agent to remove
snat/qrouter namespace.

There were other places in the code
that needed to examine the snat binding table to
check if updates were required -
validate_agent_router_combination() and
check_agent_router_scheduling_needed().

Additionally, schedule_routers() was made optional
within the rpc _notification path since it can
override the manual move being attempted.

Change-Id: Iac9598eb79f455c4ef3d3243a96bed524e3d2f7c
Closes-Bug: #1369721
Co-Authored-By: Ila Palanisamy <ilavajuthy.palanisamy@hp.com>
Co-Authored-By: Oleg Bondarev <obondarev@mirantis.com>