Delegate from first available compute host

The 'Drop authorized keys file script locally' task shouldn't
specifically require that the first member of the nova_compute group be
available. If the playbook is run with a limit excluding that host, the
task will always end up being skipped.

Use an intersect filter to find the first available compute host within
the play and use that as the delegation host.

Closes-Bug: 1632447
Change-Id: Id12fd6cee104a6fb705acc66704f21d629694d84

tasks/nova_compute_key_distribute.yml

@@ -13,6 +13,13 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+- name: Determine first available nova compute host
+  set_fact:
+    first_available_compute: "{{ groups['nova_compute'] | intersect(play_hosts) | first }}"
+  tags:
+    - nova-key
+    - nova-key-distribute
+
 # The authorized key file script will be generated locally and copied to all known
 #  compute hosts within the environment. This script will add a key to the nova
 #  user's .ssh/authorized_keys file if it's not already found.
@@ -22,7 +29,7 @@
     dest: "/var/tmp/openstack-nova-key.sh"
     mode: "0755"
   delegate_to: localhost
-  when: inventory_hostname == groups['nova_compute'][0]
+  when: inventory_hostname == first_available_compute
   tags:
     - nova-key
     - nova-key-distribute