adding hypervisors panel to admin dashboard

This commit adds a new panel "Hypervisors" to the system panel in
the admin dashboard listing all registered hypervisors hosts by
using the os-hypervisors extension of the Nova API.

fixes bug #1197763

Change-Id: I3f9a7858342c3592521b7fa8ecf17e2fb5af6cda
Merge "adding hypervisors panel to admin dashboard"