Tune sqlalchemy backend settings for OS on scale

- Tune SQLAlchemy backend settings for Openstack (QueuePool related)
- Evaluate sqlalchemy backend scaling parameters at osnailyfactor
- Fix deprecated in I [DEFAULT] usage for database config
- For Neutron use neutron_config[:database] as well
    * Ensure max_pool_size in [10, 30] limits (10 is config defaults)
    * Ensure max_overflow in [20, 60] limits (20 is config defaults)

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

Change-Id: I0e843022f04f53150da52858280034aad6aee987