Allow nova to use different configuration files

The idea is to give a way to specify different configuration files for the
different nova services.

If a different configuration file is provided in the
os-apply-config/etc/nova folder the related service will be started using
both the common nova.conf and the service specific file.

Change-Id: Ie35041eb38a9b9305495bdd1afd57672db2af237
Merge "Allow nova to use different configuration files"