conf: Detail the 'injected_network_template' opt
conf: Detail the 'injected_network_template' opt

Mostly based on this blog:

  http://blog.oddbit.com/2015/06/26/openstack-networking-without-dhcp/

Change-Id: Ib4c6acef2ff01d481a35b585e2225a065beb1305
Implements: blueprint centralize-config-options-ocata

rahulunair/nova,jianghuaw/nova,phenoxim/nova,gooddata/openstack-nova,mikalstill/nova,hanlind/nova,klmitch/nova,gooddata/openstack-nova,rajalokan/nova,Juniper/nova,rahulunair/nova,openstack/nova,vmturbo/nova,vmturbo/nova,rahulunair/nova,mikalstill/nova,openstack/nova,jianghuaw/nova,hanlind/nova,Juniper/nova,mahak/nova,gooddata/openstack-nova,klmitch/nova,phenoxim/nova,rajalokan/nova,rajalokan/nova,gooddata/openstack-nova,jianghuaw/nova,klmitch/nova,vmturbo/nova,rajalokan/nova,openstack/nova,Juniper/nova,vmturbo/nova,mahak/nova,Juniper/nova,hanlind/nova,klmitch/nova,mahak/nova,mikalstill/nova,jianghuaw/nova