Configurable container image

It is now possible to configure the container image used by portainer when using the docker containerizer. Unfortunately because support for `--privileged` has not made it's way into a mesos release, though will be available in the coming release, we still cannot support for the docker containerizer built into mesos.

Having said that, portainer now fully supports building images without a docker containerizer, so long as a docker daemon is running on the mesos slaves. You can set the `--docker-host` command line argument to trigger this. It goes without saying, `--docker-host` and `--container-image` are mutually exclusive.