Fix compute service collection with ceilometer
Fix compute service collection with ceilometer

The Ceilometer 'instance' metric was deprecated and is definitively
removed in Ocata [1][2]. Consequently, the compute service collection
is broken when using the ceilometer collector backend.

This patch fixes this by replacing the 'instance' metric by the
'cpu' metric.

[1] https://review.openstack.org/#/c/400973/
[2] https://review.openstack.org/#/c/418143/

Change-Id: I55d2c9a5cd88b3cc8363f394803328bf64ba6baa
Closes-bug: 1657493

openstack/cloudkitty,openstack/cloudkitty,stackforge/cloudkitty,stackforge/cloudkitty