Comment 13 for bug 1790721

Reviewed: https://review.openstack.org/600098
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=f588a0b6c337368596bc8732141e864f56f72212
Submitter: Zuul
Branch: stable/rocky

commit f588a0b6c337368596bc8732141e864f56f72212
Author: Matt Riedemann <email address hidden>
Date: Tue Sep 4 20:57:33 2018 -0400

    Fix nova-status "_check_resource_providers" check

    The way in which this check counted compute nodes was
    broken because of an incorrect for/else condition. If
    the check is run with a nova.conf like we have in
    devstack, where the API database is configured but
    the [database]/connection is pointing at cell0, where
    there are no compute nodes, the check passes saying
    there are no compute nodes even if the are compute
    nodes found in the cell databases (in the for loop).
    This is because the else executes because the for loop
    doesn't break, and then _count_compute_nodes returns 0
    for cell0 and overwrites the num_computes variable.

    This fixes the issue by checking if we have cell mappings
    before running the loop, else we hit the else block as
    was originally intended.

    Change-Id: I1a706d028a9ca894348a19b7b3df1ea673e4ec90
    Partial-Bug: #1790721
    (cherry picked from commit dcd421ae9e6f0391fea06c9d20949267354c3b3c)