fix vagrant 1.4.x sending array of ssh keys

Vagrant added the ability to have multiple private keys defined in mitchellh/vagrant#907.  In rubber, this changed the RUBBER_SSH_KEY env variable that the provisioner uses to pass through to the rubber commands, which it was just doing a .to_s on the array, resulting in an invalid path.

This changeset fixes that, and allows for multiple private keys to be passed in via the environment variable, comma separated.