Move driver unittest modules to proper place

It is expected, that unittests cover project modules in relation 1:1 and
placed in appropriate tree as main modules do.

It should be like in example:
manila/[tests/]foo/bar/[test_]modulename.py

Also fixed stub of service_instance in generic driver test suite, it was
overwriting service instance manager class for whole test run. Problem
appears only with this commit because order of tests is changed here.

Change-Id: I26136b21f55fb2563f15a9a44ca0b6613f75165c
Partial-Bug: #1348633
Merge "Move driver unittest modules to proper place"