The last time I was working with Ubuntu MaaS and using Juju the original examples and documentation suggested that you minimally needed five nodes to spin up OpenStack, as I recall. What I wanted to do was to put several services onto a bigger computer and then use the many smaller computers to be left over for other things.
I was working through this documentation, for what it's worth.
I was able to re-create the steps a little to target LXC rather than using up the entire node for only one service.
// Start deploying to node 0
* juju deploy --to lxc:0 mysql
Added charm "cs:trusty/mysql-28" to the environment
* juju deploy --to lxc:0 rabbitmq-server
Added charm "cs:trusty/rabbitmq-server-34" to the environment
* juju deploy --config=openstack.cfg --to lxc:0 keystone
Added charm "cs:trusty/keystone-28" to the environment
* juju deploy --config=openstack.cfg --to lxc:0 nova-cloud-controller
Added charm "cs:trusty/nova-cloud-controller-60" to the environment
Doing this, I was able to split the heavy lifting across one or two of the bigger nodes and then leave the rest for nova-compute as I recall.