The logic to handle dnsmasq hostfiles is moved from ironic-inspector
with only cosmetic changes. The logic to purge the hostsdir is not
copied since it relies on running commands with root privileges.
A documentation example is added instead.
The change is missing the RPC call to notify the filter about changes.
It will be done in a follow-up.
Change-Id: Ie32018c760c39873ead1da54cfaeae87eaaaf043
89fe0396af