Refactored unit tests structure

Created SavannaTestCase class with utility methods. It allows to:
  * set up context for the test (set up automatically in setUp, can be overwritten)
  * override savanna config for the test

Created SavannaWithDbTestCase that sets up db automatically in setUp. It replaces old DbTestCase.

Closes-Bug: #1273457

Change-Id: I429f43df30bf12e6801a6c8f1bf00d9b09c8d15c
Merge "Refactored unit tests structure"