Define cache_timeout variable

Much like the 'debug' variable, the 'cache_timeout' variable
is an expected variable, but is currently not defined in the
role. This causes the role to fail when the galera_client
meta-dep is removed (because that role defined it, this role
inherited it).

Change-Id: Ieb2bedd3a6ebd005f224a26010066c8d6f7928ef

vars/ubuntu.yml

@@ -13,6 +13,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+cache_timeout: 600
+
 #: Necessary packages
 gnocchi_distro_packages:
   - apache2