Fix linux dhcp unit test regression

The timing of the 1.0.8 and 1.0.9 merges left two tests in a broken
state.  One should have been removed entirely in
https://review.openstack.org/#/c/114328/ (and our 006f9af) and the
other should have been corrected by the introduction of a constant
in b75c681.  This patch rectifies both issues.

Closes-Bug: #1250644
Closes-rally-bug: DE872
Upstream-Review: https://review.openstack.org/#/c/114328/
Backported-from: Icehouse