Missing environment variable OS_BOOTSTRAP_PASSWORD

Issue

During a minor update of the Overcloud, the update fails with the following message visible in the output of openstack stack failures list --long overcloud:

            "Running container: keystone_bootstrap", 
            "keystone", 
            "$ docker exec --user=root keystone /usr/bin/bootstrap_host_exec keystone keystone-manage bootstrap", 
            "Either --bootstrap-password argument or OS_BOOTSTRAP_PASSWORD must be set.", 
            "Error running ['docker', 'exec', '--user=root', u'keystone', '/usr/bin/bootstrap_host_exec', 'keystone', 'keystone-manage', 'bootstrap']. [1]", 
            "stdout: Either --bootstrap-password argument or OS_BOOTSTRAP_PASSWORD must be set.",