Fail fast when required secrets are not present

Required variables that are secrets (i.e. passwords and keys) are validated as
present before any part of the role executes.

Change-Id: I91a7130005101006bb219f2911b4c38626786908

defaults/main.yml

@@ -390,3 +390,9 @@ keystone_keystone_conf_overrides: {}
 keystone_keystone_default_conf_overrides: {}
 keystone_keystone_paste_ini_overrides: {}
 keystone_policy_overrides: {}
+
+keystone_required_secrets:
+  - keystone_auth_admin_password
+  - keystone_container_mysql_password
+  - keystone_rabbitmq_password
+  - keystone_service_password

tasks/main.yml

@@ -13,6 +13,14 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+- name: Fail if our required secrets are not present
+  fail:
+    msg: "Please set the {{ item }} variable prior to applying this role."
+  when: (item is undefined) or (item is none)
+  with_items: keystone_required_secrets
+  tags:
+    - always
+
 - name: Gather variables for each operating system
   include_vars: "{{ item }}"
   with_first_found: