Don't store Nova DB credentials on compute nodes

Remove NovaDSN from overcloud compute.

When using the Conductor the Nova compute service
does not need access to the database. This patch
removes all references to the Nova DSN in the overcloud
compute templates.

Change-Id: If75f480489b84002dd061c183dbee3572a8b63f1
Merge "Don't store Nova DB credentials on compute nodes"