Add HA support to the l3 agent

* Add HA mixins used by RouterInfo and LNAT3Agent
* For HA routers: Internal, external and floating IP addresses are no
  longer configured by the agent. Instead the interfaces and addresses
  are passed to a keepalived configuration, which configures the
  addresses when the router transitions to the master state.
* Only the master instance of the router opens the metadata proxy.
  This happens due to keepalived notification scripts that are
  called upon state transitions.
* Extra routes are handled via keepalived virtual routes and are
  no longer configured by the agent.
* HA routers create a 'HA device' on a VRRP-traffic only HA-network.
* Functional testing: Add two new tests to the L3 agent:
    1) Translation of a router configuration to a keepalived
       configuration.
    2) HA specific events when creating a HA router - Assert that
       keepalived is up, etc.

Change-Id: I83f2a5d2af42164c42773b385ba7b00872eed54e
Implements: blueprint l3-high-availability
Co-Authored-By: Assaf Muller <amuller@redhat.com>