Added DB migrations on Alembic

This commit migrates Murano database migration framework from
sqlalchemy-migrate to Alembic. sqlalchemy-migrate is considered abandoned. All
other OpenStack projects are in  process of migration to Alembic or have
already finished migration.

This change doesn't preserve all the migration scripts, it merges them into a
single migration script. That's what we would do even without Alembic. It's a
common practice to squash migration scripts into a single one right after the
release.

NOTICE:
CLI command to run migrations changed. Now it looks like this:
$ murano-db-manage upgrade

Migration path:
1. Simple solution is just to re-create your database from scratch
2. You can stamp your database with revision to make Alembic think that it
   already applied the first migration:
   $ tox -e venv -- murano-db-manage --config-file etc/murano/murano.conf stamp --revision head

For usage instructions see:
murano/db/migraiton/alembic_migrations/README

This patch also adds opportunistic tests for migrations. These tests will run in
OpenStack Infrastructure jenkins slaves. It'll use real MySQL and Postgres databases.
Tests will run in a "snake walk" manner, which means that upgrade and downgrade path
will be tested.
Base for migration tests is copied from Nova. Please note, that at this moment we
cannot use code from oslo, because it is not documented and is not tested. Once
test_migrations_base appears in project 'oslo.db', we'll be able to remove our own
test_migrations_base and use the one from 'oslo.db'.

implements: blueprint alembic-migrations
Change-Id: I5aa978f7095efc57f2d6fad81b5553e1880ad931
Merge "Added DB migrations on Alembic"