Add Octavia loadbalancer dashboard

Adds a dashboard for Octavia loadbalancers. Makes use of the
openstack_loadbalancer metrics that are provided by the openstack
exporter. Has a combined table to show metrics for both loadbalancers
and amphorae.

Co-Authored-By: Matt Anson <[email protected]>
Change-Id: I4773503eb2b7e1f628656e3833d5d268402f5c73

Author: technowhizz