docker-machine on an existing, "generic" host?
Solution 1:
docker-machine can do it out of the box since version 0.3 using the generic driver which allows to add existing VM/host:
$ docker-machine create -d generic \
--generic-ssh-user ubuntu \
--generic-ssh-key ~/Downloads/manually_created_key \
--generic-ip-address 12.34.56.78 \
jungle
https://docs.docker.com/machine/drivers/generic/