Optimise apt cache update task

In https://review.openstack.org/457391 we consolidated
the package install tasks into one set, but it's also
possible to provide the package action plugin the
parameters for the underlying module, so we do that
here to get rid of the apt cache update task.

The minimum Ansible version is raised to 2.2 due to a
known bug [1] in Ansible's apt module which does not
update the cache properly if the cache update and the
install are combined in a single task.

[1] https://github.com/ansible/ansible-modules-core/issues/1497

Change-Id: If3209379c04b3bfdf3fd8af8615fc6e0036b1c52

meta/main.yml

@@ -18,7 +18,7 @@ galaxy_info:
   description: Installation and setup of memcached
   company: Rackspace
   license: Apache2
-  min_ansible_version: 2.0
+  min_ansible_version: 2.2
   platforms:
     - name: Ubuntu
       versions:

tasks/memcached_install.yml

@@ -13,17 +13,12 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-- name: Ensure apt cache is up to date
-  apt:
-    update_cache: yes
-    cache_valid_time: "{{ cache_timeout }}"
-  when:
-    - ansible_pkg_mgr == 'apt'
-
 - name: Install memcached packages
   package:
     name: "{{ item }}"
     state: "{{ memcached_package_state }}"
+    update_cache: "{{ (ansible_pkg_mgr == 'apt') | ternary('yes', omit) }}"
+    cache_valid_time: "{{ (ansible_pkg_mgr == 'apt') | ternary(cache_timeout, omit) }}"
   register: install_packages
   until: install_packages | success
   retries: 5
@@ -34,6 +29,8 @@
   package:
     name: "{{ item }}"
     state: "{{ memcached_package_state }}"
+    update_cache: "{{ (ansible_pkg_mgr == 'apt') | ternary('yes', omit) }}"
+    cache_valid_time: "{{ (ansible_pkg_mgr == 'apt') | ternary(cache_timeout, omit) }}"
   register: install_test_packages
   until: install_test_packages | success
   retries: 5