Move dnsmasq-neutron.conf into subdir

If you try to use a custom Neutron dnsmasq conf file today many
of the Neutron servers will fail to start with the following
error messages:

oslo.config.cfg.ConfigFileParseError: Failed to parse
/etc/neutron/dnsmasq-neutron.conf: at
/etc/neutron/dnsmasq-neutron.conf:1, Section must be started before
assignment: None

In this patch we move the custom dnsmasq-neutron.conf into
a 'dnsmasq' subdirectory so that the --config-dir options
for our neutron services no longer pick it up by default.

Change-Id: I120fad69a068811403a460507b6b0769e764742f
Closes-bug: #1305254
Merge "Move dnsmasq-neutron.conf into subdir"