Edits on section_neutron-single-flat

Update, as suggest by Matt in https://review.openstack.org/#/c/66231:
* Use lowercase database
* Use <replaceable>controller</..
* Fix another wrong usage of sql_connection

Change-Id: Id2946e280b041ecf2e7b228ff0974997ae7545a9
Merge "Edits on section_neutron-single-flat"