Docs: update the path of "meters.yaml" and its new feature
Docs: update the path of "meters.yaml" and its new feature

The file has been moved from ceilometer/meter/data/meters.yaml to
ceilometer/data/meters.d/meters.yaml, in order to support loading
multiple meter definition files.

So I think it is necessary to update the info in doc.

Change-Id: I6e2dbe36cd6a3cf2c198dfbdffe696a954a57820
Related-Bug: #1479775

openstack/ceilometer,openstack/ceilometer