Tune mysql, galera, sqlalchemy backend settings

- Tune mysql+galera settings
- Tune SQLAlchemy backend settings for Openstack

Closes-bug: #1274784
Implements blueprint: optimize-db-performance-at-scale

Change-Id: I0e843022f04f53150da52858280034aad6aee987
Signed-off-by: Bogdan Dobrelya <bdobrelia@mirantis.com>