Move sqlalchemy migration from Nova

Added functions for work with migration scripts to
openstack/common/db/sqlalchemy/migration.py file. This code can be used
in Nova, Cinder, Glance, etc. Added new DbMigrationError exception to
openstack/common/db/exception.py file.
Tests added.

blueprint test-migrations

Change-Id: Ia63f7fb89b0a6baacf3fd424c0db09c74680af4a
Merge "Move sqlalchemy migration from Nova"