Use hypervisor hostname for compute trust level

In XenAPI, service hostname and compute node hostname is different
because the Nova compute service may run in a separated VM and is
different with the hostname of the compute node.

The remote attestation service use the compute node's hostname because
it's the compute node that will run the servers.

Closes-Bug: #1223452

Change-Id: I9a7ce74d595531196804615a8947e253b0bd3f1a
Merge "Use hypervisor hostname for compute trust level"