Added tox environment for gathering coverage

This technique was borrowed from the tox "cover" environment in
openstack/nova's tox.ini.  This leverages the fact that stestr lets
you override the python executable via the PYTHON environment
variable.  Doing this allows us to easily generate coverage for our
unit tests.

An important caveat is that this does not provide any coverage for
tests via zaza, amulet, etc.  It is purely focused on the unit tests.

Note that this replaces the previous .coveragerc; coverage
configuration is instead pulled from tox.ini.

Change-Id: I46e3cfc0bb5a7853325b5d9351190d3687080318