[ffwd] Disable scheduling on computes before upgrade

When testing OCP on OSP it was noticed that our automation is too
crude and does not disable scheduling on upgraded computes. This
results with few dead workloads that got scheduled on the compute
right before the upgrade.

Change-Id: I02243028a5ba3c6450e7633740fcfd757d9b20b6
Resolves: UPG-2447

tasks/fast-forward-upgrade/overcloud_upgrade_hosts.yaml

@@ -11,6 +11,15 @@
   when: ffu_upgrade_workarounds
   tags: ffu_overcloud_system_upgrade
 
+- name: Disable scheduling on {{ host }}
+  when:
+    - compute_present|bool
+  shell: |
+    set -o pipefail
+    source {{ overcloud_rc }}
+    HOST="$(openstack compute service list -f value -c Host | grep {{ host }})"
+    openstack compute service set --disable --disable-reason FFWD "$HOST" nova-compute
+
 - name: run the pre upgrade script for the host {{ host }}
   when:
     - compute_present|bool
@@ -59,6 +68,15 @@
     timeout: 600
 
 ######## overcloud upgrade run ########
+- name: Enable scheduling on {{ host }}
+  when:
+    - compute_present|bool
+  shell: |
+    set -o pipefail
+    source {{ overcloud_rc }}
+    HOST="$(openstack compute service list -f value -c Host | grep {{ host }})"
+    openstack compute service set --enable "$HOST" nova-compute
+
 - name: apply pre ffu overcloud upgrade workarounds in {{ host }}
   shell: |
     set -o pipefail