Migrate from common code to oslo.db

Migrates Tuskar from common code to olso.db
Adds oslo.db to requirements.txt.
Adds oslotest to test-requirements.txt.
Adds EngineFacade lazy loading - inspired in ironic.
Updates imports and calls to adapt to moved code in oslo.db.
Updates import_opt calls parameters to reflect move configs.
Adds note to docs about setting up database.connection.
Regenerates tuskar.conf.sample.

Change-Id: I709209bc8ce82b7134d738da3a0fb609d02ffb8c
Closes-Bug: #1290849
Merge "Migrate from common code to oslo.db"