Added base for functional tests

This change allows to run functional tests with in-memory sqlite DB.
Tests will use DB schema memoization. Here is the algorithm:
1. Statically create global DB fixture
2. Run migration script (only once)
3. Copy DDL and save it in the global state
4. Run DDL to initialize DB for each test

This algorithm allows to skip long-running migration scripts and initialize
database in one step.

Based on openstack/ironic tests.

Co-Authored-By: Ruslan Kamaldinov <rkamaldinov@mirantis.com>

Change-Id: If80abd1b616548495d3466037aef54ade8f1614d