Refactoring neutron db config opts to be in neutron/conf/db/ and
neutron/conf/agent/database/ so that all the configuration options
reside in a centralized location. This simplifies the process of
looking up the config opts and provides an easy way to import.
Change-Id: I4da9bb48d49b99e8c2b34a5c1b83e7eb95b70b82
Partial-Bug: #1563069