The kubeadm preflight looks if the docker service is enabled,
rather than checking the docker socket... So make it happy.

This adds an environment variable file for crio.service to source.
And a step for the buildroot provisioner to write flags, initially just
insecure-registry.
Signed-off-by: NVincent Batts <vbatts@hashbangbash.com>

The current implementation assumes that we already have docker running.
 This switches it to not remove any previous docker configuration
(since there isn't any), and uses our native file transfer utils
instead of the printf commands.

docker/machine made a change (https://github.com/docker/machine/pull/3966) to use a systemd drop-in unit (https://coreos.com/os/docs/latest/using-systemd-drop-in-units.html) instead of overwriting the dockerd configuration. This necessitates clearing the ExecStart list inherited from the base config prior to the ExecStart directive in GenerateDockerOptions -- otherwise, docker.service fails to start with log output like "docker.service: Service has more than one ExecStart= setting, which is only allowed for Type=oneshot services. Refusing."

Sometimes the docker daemon isn't ready and on a restart and libmachine
times out while trying to reach it.  This retries when it isn't ready.

This fixes timeout problems in our virtualbox integration tests.