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* docker.service was out-of-date with the default CoreOS copy (and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* fleet.service was out-of-date, and our change was never being applied (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 the `start` just took affect)

TESTING: Provisioning should work just as it did before
Merge pull request #1464 from deis/cleanup_userdata

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