demote f20 to experimental

as we're currently at quota, with about 1.5x quota worth of load
in the check queue. That's put us in a state that f20 nodes are
not being allocated at all. Which means that devstack and devstack-gate
changes are currently stalled.

The fix for the top gate bug is probably a devstack fix. Which puts us
in a position where we can't make things better because we're
stuck waiting for f20 nodes.

Move to experimental for now. We need a longer term fix to prevent
us from getting into this situation before moving it back to check.

Change-Id: I7f44cb9896f646060d45ac571ddb000b721812a6

modules/openstack_project/files/zuul/layout.yaml

@@ -2395,7 +2395,6 @@ projects:
       - check-tempest-dsvm-ironic-nv
       - check-tempest-dsvm-ironic-postgres-nv
       - check-tempest-dsvm-virtual-ironic-nv
-      - check-tempest-dsvm-f20
     gate:
       - gate-devstack-bash8
       - gate-devstack-dsvm-cells
@@ -2411,6 +2410,7 @@ projects:
       - gate-grenade-dsvm-forward
       - check-tempest-master-dsvm-full-havana
       - check-tempest-master-dsvm-neutron-havana
+      - check-tempest-dsvm-f20
 
   - name: openstack-dev/grenade
     check:
@@ -2592,7 +2592,6 @@ projects:
       - check-tempest-dsvm-ironic-nv
       - check-tempest-dsvm-ironic-postgres-nv
       - check-tempest-dsvm-virtual-ironic-nv
-      - check-tempest-dsvm-f20
     gate:
       - gate-devstack-gate-unittests
       - gate-devstack-dsvm-cells
@@ -2604,6 +2603,7 @@ projects:
       - gate-grenade-dsvm-forward
       - check-tempest-master-dsvm-full-havana
       - check-tempest-master-dsvm-neutron-havana
+      - check-tempest-dsvm-f20
     experimental-tripleo:
       - check-tripleo-ironic-seed-precise
       - check-tripleo-undercloud-precise