Configure nova-network database access on debian/ubuntu

The database configuration disappeared from the debian/ubuntu
installation guide. Add the configuration bits in nova.conf, as done for
other distributions.

Change-Id: I7f7c754c7ff8d6898149507b49efd5a9d51e7ef0
Closes-Bug: #1244216
Backport: havana
(cherry picked from commit c308a59cffd8a453c0b566686689556dcf68c90d)