Not count disabled compute node for statistics

No server will be scheduled to disabled compute service, thus we
should not count the corresponding compute node information.

It's arguable if we should count for 'down' service, since service
may be marked down because of communication error. If we do want to
exclude the down service, we need passing the information from caller
because the up/down state is not kepts in database, and it means compute
and cell api changes.

Closes-Bug: #1285259

Change-Id: I5e3e71ef30683c5eb5cc4462f58fa5f29d7c3f4b
Merge "Not count disabled compute node for statistics"