Add a standard for classifying containers

[llunved]

In practice there are different kinds of container images:

• Basic bootstrap such as a base image and runtimes such JBoss EAP, Python, etc. These images are meant for building upon.
• There are individual service components such as e.g. a mariadb image delivered by an ISV, or a db-cluster atomic-app image. These are meant to be aggregated into applications, but by themselves usually would not be modified beyond parameterization.
• We have top-level applications, that just will be parameterized and the run - e.g. a containerized delivery of Kolab. Could be a single, multi-service container for an existing enterprise app or a full atomic-app. These ususally will just be installed, parameterized and run.
• There sometimes are containers that are simply 'hidden' components of something else - e.g. a component of an atomic-app or something like the openshift pod container, that is not useful on it's own and would only be used when pulled in by something else.

It would be a good thing to capture this classification in the meta-data to e.g. be able to filter search on it.

[rhatdan]

Makes sense, but not sure what a generic name/label for this would be.
container_class
Just
class