Log dnsmasq host file generation

Log dnsmasq host file generation

We've been seeing things that appear to be races between the hosts
files being written out for dnsmasq and dhcp requests coming in.  We
will get occasional errors from dnsmasq saying "no address available",
"duplicate IP address" but by the time you look, the corresponding
host file has long since been replaced.

If we had some debugging like this, we could at least correlate what
neutron thought dnsmasq knew at the time the requests were coming in.

We store the filename to avoid multiple lookups, and also
s/name/filename/ to avoid ambiguity and be more consistent with the
rest of the code.

Closes-Bug: #1294892
Change-Id: Ifa92ca71419ce0155b28d2015eff9e82557a0825