Tune sqlalchemy backend settings for OS on scale

- Tune SQLAlchemy backend settings for Openstack (QueuePool related)
- Fix deprecated in I [DEFAULT] usage for database config
- For Neutron use neutron_config[:database] as well

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

Change-Id: I0e843022f04f53150da52858280034aad6aee987