If none of the floating IPs listed within Horizon are attached to an
instance, do not request a list of Nova instances (because we have no
use for it). Apply the same behavior for Volumes tab (both Admin and
Project). This patch both saves us from making unnecessary and
potentially expensive call to Nova and lays the ground for a more
selective request of instance details from Nova. The latter will be
possible once Nova supports filtering instance details by >1
instance_id. For reference see
https://blueprints.launchpad.net/nova/+spec/get-multi-servers-filter
Change-Id: Ie7563b9e03b286b4cf51507463213162af3383b1
Partial-Bug: #1442310