Implement migration of legacy routers to distributed

Implement migration of legacy routers to distributed

This patch addresses the necessary changes required
for the router migration or conversion from legacy
to distributed.

The _update_router_db() method was modified to add
logic to support the SNAT functionality of DVR by
calling create_snat_intf_ports_if_not_exists().  A
call to _unbind_router() was also called to force
the scheduling of the router with 'distributed'
enabled.

Right now we will be supporting migration from legacy
to distributed only.

Closes-bug: #1348309
Partially-implements: blueprint neutron-ovs-dvr
Co-Authored-By: Michael Smith <michael.smith6@hp.com>
Change-Id: I6f252271bb5b52ce57184ad7b0bf1ce280b965fb