Move tests into manilaclient package

tests/__init__.py implies a package in the global namespace called tests.
That's not what these are, they are tests in the manilaclient namespace.

Also moved unit tests to dir 'unit', it is done so because 'functional' tests
are expected to be added.

Change-Id: I801214b865e9cef8db7511f209dcf3c26b75d70f
Merge "Move tests into manilaclient package"