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