Fixes Gnocchi & external Ceph integration

The removal of Kolla Ceph deploy [1] broke gnocchi & external Ceph
integration - the variable gnocchi_pool_name is referenced in the config
template, but should now be ceph_gnocchi_pool_name.

This change fixes the issue.

Reported by Nick Wilson.

[1] https://review.opendev.org/#/c/704309/12/ansible/roles/gnocchi/defaults/main.yml

Change-Id: I7089781c0c4d7bce8a44cb8b1fca847dd0b7efd1
Closes-Bug: #1877974

ansible/roles/gnocchi/templates/gnocchi.conf.j2

@@ -74,7 +74,7 @@ driver = file
 file_basepath = /var/lib/gnocchi
 {% elif gnocchi_backend_storage == 'ceph' %}
 driver = ceph
-ceph_pool = {{ gnocchi_pool_name }}
+ceph_pool = {{ ceph_gnocchi_pool_name }}
 ceph_username = {{ ceph_gnocchi_user }}
 ceph_keyring = {{ ceph_gnocchi_keyring }}
 ceph_conffile = /etc/ceph/ceph.conf