Refactored fake connection URL classes

Changed the way how DB tests are run to make them coope with concurrent
databases.
FakeUrl -> DbManager.
DbManager becomes a fixture that is able to create a separate DB
for each test and perform a cleanup action if needed (delete database)

This is a preparational work for adding new database backends.

Related to blueprint sql-unit-tests-on-real-backend

Change-Id: I7cbaa084df4a69c453347a45d884e8e808ac697d