Replaces call to Gnocchi status for a request of metric list when determining...

Replaces call to Gnocchi status for a request of metric list when determining backend in openstack vnf collector

When trying to determine metric backend, the openstack vnf collector was
requesting Gnocchi status data, which is only available for privileged users.
This generated errors in the metric collection when using a nonadmin openstack user.

Change-Id: I6a85f27c4714a1d3f8bbbf82da1795066352a1e0
Signed-off-by: Benjamin Diaz <bdiaz@whitestack.com>