Add f20 job to devstack-gate

Add f20 job to devstack-gate.  With [1] this job becomes a check job
for devstack, so we should be checking changes to the devstack-gate
scripts on f20 to ensure this test remains stable.

[1] https://review.openstack.org/#/c/98280/

Change-Id: I69f2c7584ebe01721c68eb095552c167c3a96422
Merge "Add f20 job to devstack-gate"