Move nova.conf/neutron.conf database configuration variables to [database] section

Commit https://review.openstack.org/34671 introduced for nova.conf:
"The database configuration variables are now in a new section
 [database]."

This change is also needed for the neutron files.

Change-Id: I3dcc46c163560e6ca277d421acf87a75683fa83d
Partial-Bug: #1205060
Merge "Move nova.conf/neutron.conf database configuration variables to [database] section"