The Docker 1.9.1 package on Debian is broken, and the service fails to
install when run unattended. This is treated as an installation failure
and causes everything to fail.
However, the service can be started by Salt once we're not installing
the package, and indeed we restart docker anyway.
So, on Debian, use a helper script to install the docker package. The
script sets up a policy-rc.d file to prevent the service starting, and
then cleanly removes it afterwards (this would be difficult to do in
Salt, I believe).
