allow additional user-defined userdata/cloud-config

On ec2 (and, in the future, openstack), there are various tasks I like completed via userdata and/or cloud-init on all instances I launch. I should be able to do the same when launching via ensemble. It would be great if the various ensemble commands that launch instances took an argument (similar to euca-run-instances) to specify user data to include in addition to what ensemble already generates. Additionally, a configuration option in environment.yaml to point to a file to be included in all launches would great as well. It might be necessary to do some sanity checking to ensure there are no conflicts with what ensemble requires for bootstrapping, but AFAIK cloud-init is already flexible enough.