implement dynamic checks using rootless podman

[TomasTomecek]

Let's implement them using rootless containers which were recently merged into podman:

https://github.com/projectatomic/libpod/pull/936 podman-0.6.5+

This means that backend code for dynamic checks would utilize podman tool and the container would be invoked with an unprivileged user. podman would create and run the container using user namespaces. This means that user namespaces need to be set up correctly (/etc/subuid). These checks need to run in an openshift pod (that's how we invoke colin internally).