chore(contrib/coreos): clean up user-data

Our user-data has some unnecessary cruft in it:
* docker.service was out-of-date with the default CoreOS copy (and actually was never really being applied anyhow).
* docker-tcp.socket is no longer used - docker.socket is defined by CoreOS
* fleet.service was out-of-date, and our change was never being applied (I think just the `start` took affect)

TESTING: Provisioning should work just as it did before