Tune mysql, galera, sqlalchemy backend settings

- Tune MySQL config settings to be more clouds-specific (reads much
  more common, reads:writes rate is 10(100):1)
  * Limit innodb_buffer_pool_size to 10G
  * Set innodb_log_file_size as 25% of innodb_buffer_pool_size (max 2G)
  * Reduce innodb_write_io_threads
  * Use low_priority_updates
  * Raise FD imits to 102400, max connections, open tables limits etc.
- Tune SQLAlchemy backend settings for Openstack (QueuePool related)
- Sync some MySQL+galera class params for MySQL class as well

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

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