Clarify sql string when not using nova-conductor

You do not have to use nova-conductor and can
instead use direct DB connections from the compute
nodes. This change makes a note and clarifies the
nova.conf changes which are required if you wish to
do so.

Change-Id: I60baf1b235172ef4ea8152b0119543d17e07cd42
Merge "Clarify sql string when not using nova-conductor"