Enable fullstack multinode tests, add L3 HA test exemplar

Enable fullstack multinode tests, add L3 HA test exemplar

* Created a 'resources' subdir and moved all fixture files
  to it.
* Split ML2ConfigFixture to the server-side ml2 configuration
  fixture, and the OVS agent configuration fixture.
* Neutron process logs were using H:M:S format as their file name,
  but when starting multiple agents of the same type my machine
  was fast enough to do that in the same second so that different
  processes were outputting to the same log file. No good!
  Added ms to the log name format. I also changed the log time
  from UTC to local timezone.
* Renamed and moved 'FullstackFixture' to neutron/tests/fullstack/
  resources/environment.Environment
* Added a 'Host' abstraction that groups agents that report with
  the same 'host' value. Hosts may be interconnected by the
  environment via shared bridges.
* The 'Environment' class will accept global
  attributes (This will be later filled with stuff like tunneling,
  l2pop or other environment-level flags), and in this patch accepts
  a  list of host attributes (Configuration that may differ between
  hosts like the l3 agent mode [legacy, dvr, dvr_snat]).
* Made OVS agent and L3 agent fixtures expose their bridges
  so that I could interconnect them.
* Added a super simple L3 HA test to show that this entire thing
  works.

Change-Id: Ie64de9f35bd6ab7cbad494061613ecf5e0ccd806