Switch on coverage support for unit tests

After we merge this patch we can use 'tox -e cover' and see how is our
code is covered with tests. It's very good for proper code process.

Change-Id: Ia39145727b0dc55879cd807e2ef4fc2de331c9a4
Implements: blueprint coverage-for-testr
Nova Aggregate wrapper to manage host allocation pools

Change-Id: Ife4949896c1679c941df5b7059a8fbcbf55ec287
Yuriy review

Change-Id: I42ed935878d52c4257b1cc868baa96100f542005
fix typo

Change-Id: I16f0dda16c60a2b6888088a5053943c23553efc4