Update port bindings for master router

Update port bindings for master router

An HA port needs to point to the correct host (where the master router
is running) in order for L2Population to work.

Hence, this patch introduces two fixes:
* When a port owned by an HA router is up we make sure it points to the
  right node where the master is running, or a random node if there is
  no master yet (This corner case is fixed by the 2nd bullet point).

* When a L3 agent reports it's hosting a master, we need to update the
  port binding to the host the master is now running on. This fixes
  both routers with no elected master (Yet) and failovers.

This patch also changes the L3 HA failover test to use l2pop.
Note that the test does not pass when using l2pop without this patch.

Closes-Bug: #1365476
Co-Authored-By: Assaf Muller <amuller@redhat.com>
Change-Id: I8475548947526d8ea736ed7aa754fd0ca475cae2