Test HA router failover

Test HA router failover

HA failover testing requires the ability for two L3 agents to
co-exist on the same machine. The following changes were made
to support this:

* Add l3 agent ability to simulate nested namespaces by suffixing
  each router namespace with '@host'. For example, if a router
  with id '1' is created, its namespace will be:
  'qrouter-1@host'
* Added test.common submodule which will hold common code which is
  shared between the different kind of tests (unit and funtional).
* Added tests.common.agents submodule which holds test-only agents, and
  added a test L3NAT agent to be used in the HA functional testings.

Co-Authored-By: Assaf Muller <amuller@redhat.com>
Closes-bug: #1374946
Change-Id: I2185fe276d9d32b34c407396e7ed5d6e2801340c