Move devstack f20 jobs to f21

F21 nodes are up and running in both hp and rackspace, and
experimental runs of this test show it works.

Change-Id: I99bf07443298c4ab80702bda5b276c85e07a24e2

zuul/layout.yaml

@@ -2920,7 +2920,7 @@ projects:
       - check-neutron-dsvm-functional
       - check-swift-dsvm-functional
       - check-tempest-dsvm-ironic-pxe_ssh
-      - check-tempest-dsvm-f20
+      - check-tempest-dsvm-f21
       - check-tempest-dsvm-centos7
       - gate-devstack-publish-docs
     gate:
@@ -2937,7 +2937,6 @@ projects:
       - check-tempest-dsvm-cells
       - check-tempest-dsvm-mongodb-zaqar
       - gate-grenade-dsvm-forward
-      - check-tempest-dsvm-f21
 
   - name: openstack-dev/devstack-vagrant
     template:
@@ -3091,7 +3090,7 @@ projects:
       - check-tempest-dsvm-neutron-full-juno
       - check-tempest-dsvm-postgres-full-juno
       - check-tempest-dsvm-ironic-pxe_ssh
-      - check-tempest-dsvm-f20
+      - check-tempest-dsvm-f21
       - check-tempest-dsvm-centos7
     gate:
       - gate-devstack-gate-unittests