In Rackspace env a bigger partition is available and configured
as /dev/xvde1 but not mounted.
We should use that as working dir for the metal3 job as the
standard one is too small and the job fails regularly when
running there.
This allows to make the metal3 job voting again.
Change-Id: I4d50b9c07367a5b1ad25887d87cf2e29ac6b4257
b25a3cad60