Add root ca cert to docker-machine

When creating new docker-machine Docker nodes, one needs to add necessary root CA certificates to them in order to let them communicate with SSL protected services.

The easiest, if manual, method is to copy the ca certificate (in .pem format) to /var/lib/boot2docker/certs on EVERY swarm node (if running in swarm mode).

$ docker-machine ssh <docker-machine name> "mkdir /var/lib/boot2docker/certs"

$ docker-machine scp ca.pem <docker-machine name>:/var/lib/boot2docker/certs/some-speaking-name.pem