Fix TemplateSyntaxError at hypervisors view

If somehow nova API service-list isn't available, listing
hypervisors at /admin/hypervisors/ ends with TemplateSyntaxError.

Expected result is that the page should load but user will
see empty list and error message.

This fix is based on idea found in other views:

openstack_dashboard/dashboards/admin/...
images/views.py
aggregate/views.py
falvors/views.pu

that on any exception while getting data (get_data), we just
return empty list and handle exception to display error message.

Closes-Bug: #1392316

Change-Id: Ia665111185abecab6a96e6c72a6088cf2fa2e02b
Merge "Fix TemplateSyntaxError at hypervisors view"