Add tox -e functional

Provide a tox -e functional stub, as a place for functional tests to
migrate to. This will allow us to write a zuul test configuration for this
environment.

Part of bp:functional-tests-for-nova

Change-Id: I8a1107beab151f9bf574cfbae1d4dc18400fd74e
move all tests to nova/tests/unit

As part of the split of functional and unit tests we need to isolate
the unit tests into a separate directory for having multiple test
targets in a sane way.

Part of bp:functional-tests-for-nova

Change-Id: Id42ba373c1bda6a312b673ab2b489ca56da8c628