Expose "last_seen_up" when listing services

In response to bug #1420848 git commit https://review.openstack.org/163060 proposes a new "last_seen_up" field in the service DB model, to be used by the servicegroup DB driver to track when the service reported itself as up (as opposed to when the DB row was updated, which could happen due to other reasons.

Assuming something like that gets merged, this blueprint is simply to add the "last_seen_up" field as a property for each service in the response to a GET on /v2/{tenant-id}/os-services. The rationale for doing this is that it's possible we could end up with a recent "updated_at" timestamp due to setting the service disabled/enabled, but the service is considered as "down" due to not reporting recently. Without exposing "last_seen_up" we can't tell why it's considered "down".

If the value of "last_seen_up" is None, then we will set the value in the property to "-" to indicate that there was no meaningful information present.

It's a simple change but since it's an API change a spec is needed.