With Docker CE, the daemon sets the default policy of the iptables
FORWARD chain to DROP. This causes problems for provisioning bare metal
servers when ironic inspector is used with the 'iptables' PXE filter.
It's not entirely clear why these two things interact in this way,
but switching to the 'dnsmasq' filter works around the issue, and is
probably a good move anyway because it is more efficient.

We have added a migration task here to flush and remove the ironic-inspector
iptables chain since inspector does not do this itself currently.

Change-Id: Iceed5a096819203eb2b92466d39575d3adf8e218
Closes-Bug: #1823044