Add the var bootstrap_host_mongodb_users to allow the MongoDB user
creation tasks in the bootstrap-host role to be skipped using the
BOOTSTRAP_OPTS environment variable defined in the script
bootstrap-aio.sh.
The default value for bootstrap_host_mongodb_users is the value of
bootstrap_host_mongodb_service.
Without this change setting bootstrap_host_mongodb_service=no causes the
bootstrap-host role to fail on the task 'Add ceilometer database user'.
Change-Id: Iab30ed29f1bc1a922e5adf89b1057c598ec067e3
318109d0ed