Move dnsmasq-neutron.conf into a subdir

Update our docs to suggest that the optional dnsmasq-neutron.conf
live in a subdirectory called 'dnsmasq'. This fixes potential
issues in following these instructions where Neutron
services may be configured to use --config-dir option which
would cause this .conf file (which is in a dnsmasq format)
gets picked up and thus causes service startup failures.

See also TripleO bug #1305254

Change-Id: I169cc54d534ed1e91550854722071858ea0d0069
Merge "Move dnsmasq-neutron.conf into a subdir"