Add gnocchi backend precheckes for ceilometer

when set ceilometer_database_type or ceilometer_event_type
set to 'gnocchi', but gnocchi is not enabled, prechecks action
can not find it out.

Closes-Bug: #1672626

Change-Id: Iadd1c66279fed012fb7c1cfb1f98857c36bbc75f

ansible/roles/ceilometer/tasks/precheck.yml

@@ -23,3 +23,12 @@
     - enable_ceilometer | bool
     - not enable_mongodb | bool
     - ceilometer_database_type == "mongodb"
+
+- name: Checking gnocchi backend for ceilometer
+  run_once: True
+  local_action: fail msg="ceilometer_database_type or ceilometer_event_type set to 'gnocchi' but gnocchi is not enabled"
+  changed_when: false
+  when:
+    - enable_ceilometer | bool
+    - not enable_gnocchi | bool
+    - (ceilometer_database_type == "gnocchi" or ceilometer_event_type == "gnocchi")