Extended server attributes can show wrong hypervisor_hostname

When using the extended server status (EXT-SRV-STS) extension, it might
produce the wrong hypervisor_hostname on multi-node drivers like baremetal.

The db query used is incorrect and in fact not needed anymore, since the info
is already available in the instance dict itself.

This patch resolves the issue by instead using instance['node'] directly
and further removes the no longer used compute_node_get_by_host query.

Resolves bug 1135506.

Change-Id: I641c4f59a088754fe68378ae4bf217c59056b1d1