[cli-reference] add gnocchi subcommands
[cli-reference] add gnocchi subcommands

The gnocchi client has a different pattern of subcommands.
This patch explicitely specify the subcommands for the tool to work.

Change-Id: I68f1aaf8c0cedb756d529cb996592a07fd9fb4d1

openstack/openstack-doc-tools,openstack/openstack-doc-tools