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 soon, 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.