Tune down multinode concurrency
Tune down multinode concurrency

Reduce the multinode concurrency and number
of VMs so we don't run Rackspace VMs out of
disk space.

This is an issue that was reported by the
infra team.

[0]: http://eavesdrop.openstack.org/irclogs/%23openstack-ironic/%23openstack-ironic.2019-12-16.log.html#t2019-12-16T17:38:15

Change-Id: I867bac9e247605e561cc7ee4522f70ce6de368d4

openstack/ironic,openstack/ironic