Define cache_timeout by default

By default, the role fails in the task
``Install distro packages`` if the variable cache_timeout isn't
defined.

This should fix the issue, and be more consistent with other roles.

Change-Id: Ic15b9902e3b0b51040f3f32938b30a060c592e97

vars/ubuntu-16.04.yml

@@ -13,6 +13,9 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+## APT Cache options
+cache_timeout: 600
+
 tempest_distro_packages:
   - git-core
   - libpq-dev