Add EnvironmentDescription, pass it down

Add EnvironmentDescription, pass it down

* The EnvironmentDescription class describes an entire fullstack
  environment (as opposed to the currently implemented host-only
  descriptions). This will allow future patches to signify that a test
  should set up an environment that supports tunneling, l2pop, QoS and
  more.
* Now, most fullstack fixtures (config and process ones, at least),
  expect both the EnvironmentDescription for the current test and the
  HostDescription for the 'host' the config/process is on. This allows
  for easier and most robust future changes, as now adding a new
  parameter to one of the description objects doesn't mean adding that
  argument to a number of other objects which are using it.
* Changed HostDescription's default argument of l3_agent to False, since
  adding new configurations and defualting them to True forces the
  author to go through ALL the tests and explicitly turn them on/off.
  However, defaulting new configurations to False only requires
  explicitly turning them on, which we ought to do anyway.

Change-Id: Ib2f12016ba4371bfda76c82e11d0794acc759955