The default incoming database connection count is limited to 151
connections. These are not sufficient in a 100 node 3 controller
deployment to launch several heat stacks simulatenously.
Data measured on bare metal shows that 250 connections are needed
to launch one 25 VM heat stack with 3 controller nodes with 209
tasks (default enablement).
Change-Id: If60b540010d6d173a393fa91fa30cb3ba572cfc0
Closes-Bug: #1492719