This ensures that failure of a single host fails the whole play at that
task. This can avoid confusing errors such as when the task
"Assert that the nodepool private IPv4 address is assigned" fails on one
host, causing subsequent errors on other hosts.
Note that this only affects the Zuul playbooks, not Kolla Ansible's
playbooks.
Change-Id: I77a6534dd2ddd188f795e17d17a44be249d01f31