Replace fakes.FakeClient inheritence with Mock

NovaFakeClient had fakes.FakeClient as a super classes.

This dependency broke the build as the interface was not
well defined

Replace the super classe FakeClient with MagicMock and implement
the local return types as mock object

Closes-bug: 1364515
Change-Id: I746daf38f985eba824a5a0759f450766b08b2dab
Merge "Replace fakes.FakeClient inheritence with Mock"