Proposal: adding a simple logic to auto detect runtime

[chanwit]

Maybe if we could have --runtime=auto to

• firstly check for containerd, if not available then fallback to
• docker, if not available then display error.

[stealthybox]

reason for this is somewhat related: https://github.com/weaveworks/ignite/issues/390#issuecomment-527979961

Failing on containerd when docker is available is somewhat bad U/X. We should decide if this should be explicitly configured.