Updating hypervisor_hostname on launched instances.

In Grizzly instances now have a 'node' field that maps to the
'hypervisor_hostname' attribute instances returned by the API.
On cobalt-compute startup we query the hypervisor through the
vms_conn to determine this value and apply it to instances whenever
we set their host field.

Change-Id: I0be721b351c4d6429c77ed3134e642cd902509fd