Expose host UUID from libvirt through the API

This blueprint is for adding an additional 'hypervisor_uuid' detail to the hypervisor details list.

The hypervisor's UUID is useful to the ceilometer katello dispatcher plugin (https://github.com/Katello/ceilometer_katello_dispatcher), which needs UUIDs of both hypervisors and instances in order to populate katello with a map of which instances are running on which hypervisors. Katello and its backend entitlement service candlepin use UUIDs to track system information.

Additionally, the hypervisor UUID is a handy identifier for verifying which hypervisor is being referred to in the nova cli output.

I'd like to make the following changes:

 * add a hypervisor_uuid attribute to ComputeNodes model object (this includes a new column on compute_nodes)
 * populate hypervisor_uuid via the libvirt driver's existing get_host_uuid method
 * return hypervisor_uuid with other hypervisor details via the API calls under /os-hypervisors.

This feature requires no permission changes, it is adding a field to an existing API.

An example call would be to /v2/<tenant id>/os-hypervisors/<hypervisor id> which would return something like this:

{
    "hypervisor": {
        "hypervisor_hostname": "devstack.novalocal",
        "hypervisor_uuid": "7bc268e4-6a94-4e6e-9b54-b88958ae7998",
        "id": 1,
        <existing additional details here>
    }
}

CLI calls to "nova hypervisor-show <id>" would output this additional field as well, no changes to python-novaclient are needed.