Use the mangle checksum fill rule regardless to the multi_host

The checksum-fill is required even when you are running devstack,
in a nested kvm guest.
Always using the checksum-fill on all dhcp responses is safe, and does
not have significant impact.

The situation is similar when the checksum-fill first introduced to
nova.

See Also:
https://bugzilla.redhat.com/show_bug.cgi?id=910619#c6

Change-Id: I8a34d5f815f6bfef6d683965ed51a77e82af8834
(cherry picked from commit a6dd0cdf0a444e2f28528ef8e8a8d3f3b845b5d8)
Merge "Use the mangle checksum fill rule regardless to the multi_host" into stable/havana