Fail gracefully on node detail page when no overcloud

The node detail page retrieves the number of running instances from
Nova Baremetal, which makes the page fail rendering when the overcloud
nova is not accessible. I moved the query from template to
get_context_data and added some basic error handling.

Change-Id: Ie9919ba5ff6c3ae7b85f962d6fcecb5ef4e51c51
Closes-bug: #1237471