designate: Correctly match hostnames

'in' expresion matches substrings, therefore it is possible
that following expression will fail:
    inventory_hostname in groups['groupname'][0]
for example when:
    inventory_hostname = 'my-host-a'
    groups['groupname'][0] = 'my-host-a1'

The result is running task on multiple hosts rather than single one.
Such action might result in playbook failure

Change-Id: Ibe2d5ca2f9502a8140a895ab3ac7abc5076ce2ff
Signed-off-by: Maciej Kucia <m.kucia@partner.samsung.com>

ansible/roles/designate/tasks/deploy.yml

@@ -21,4 +21,4 @@
   meta: flush_handlers
 
 - include_tasks: update_pools.yml
-  when: inventory_hostname in groups['designate-worker'][0]
+  when: inventory_hostname == groups['designate-worker'][0]

ansible/roles/designate/tasks/update_pools.yml

@@ -1,4 +1,4 @@
 ---
 - name: Update DNS pools
   command: docker exec -t designate_worker designate-manage pool update --file /etc/designate/pools.yaml
-  when: inventory_hostname in groups['designate-worker'][0]
+  when: inventory_hostname == groups['designate-worker'][0]