Update [neutron] section in nova.conf for Juno

Use new parameters for Juno like it's documented here:
http://docs.openstack.org/trunk/config-reference/content/nova-conf-changes-master.html

* Use [neutron] section
* Delete the neutron_prefix when needed

Closes-bug #1391099

Change-Id: Ib63a80ea28b5aa5c033df3ec503031a99f4824c9
Merge "Update [neutron] section in nova.conf for Juno"