With this change docker images are no longer
named as: <Webapp.docker_name>:<WebappVersion.number>
but       <Webapp.docker_name>:id</WebappVersion.id>

This is only for storage, for the user we still present the image as
<Webapp.docker_name>:<WebappVersion.number>

There are multiple reasons to do that:
- this simplifies the controller design, because docker images are no
  longer replaced (once an image is committed with tag, 'id<SOMETHING>'
  it won't be modified anymore) -> thus it is no longer necessary to
  track the image state carefully (when pushing/pulling from/to the
  registry)
- this prevent reusing dangling images from a removed webapp (because we
  now have a strong guarantee that the image tags are unique)
- this will avoid nasty race conditions when we implement direct 'push'
  to the registry (because we then assign the new image id before the
  manifest is actually pushed, if a push and commit are done in the same
  time we will keep the latest one, i.e. with the highest id)
- this will make easy to implement image recovery: we can keep removed
  images in the registry for some time (eg: 1 month) before they are
  really deleted

Note: the REPLACED state is no longer transient (since we now keep the
replaced images in the db and since we may still have remaining
job/sandboxes using them). Maybe we can rename it as DELETED when we
implement #265.