Preserve consistency in module namings

We rename several modules in the system (mostly in unit tests) so that their
namings become consistent throughout the code.

"Consistency" means here following certain rules like "name all deploy engine
and server provider modules without _engine.py and _provider.py suffixes" or
"name all unit tests like test_<tested_module>.py".

We also add a couple of new folders in unit tests so that the unit tests
structure strictly resembles that of tested modules.

Blueprint unit-tests-refactoring

Change-Id: Ia7b6afce0d1e20b969b56301e0c9ad0f94cc60e4