Rename nailgun.test.base.Environment class

  In Fuel terminology we use "environment" term as a synonym for
  "cluster", but in our tests Environment class doesn't represent
  the cluster entity and behaves more like "Environment Manager",
  since it may have more than one cluster.

Change-Id: I09dab88eaca583144e68f998283ebce236d728b7
Implements: blueprint code-testing-improvements
Merge "Rename nailgun.test.base.Environment class"