support multiple container engines on the host

[BenTheElder]

#151 should bring initial support for podman via @rhatdan

This should mostly be a drop in replacement. (it's not... podman has quirks)

We should further clean up the code base a bit to help with this sort of thing. We've mostly but not quite isolated the container runtime functionality.

/assign
/assign @rhatdan

/kind feature
/kind cleanup
/priority important-longterm

related: #153 is the same thing, but within the nodes

[BenTheElder]

There's now a container package with docker as a subset, and we're working on plumbing through optional arguments to the cluster better, I expect to get limited podman support based on these pretty soon.

[BenTheElder]

We're also targeting CRI functionality explicitly: https://kind.sigs.k8s.io/docs/design/principles/#target-cri-functionality

[BenTheElder]

Currently working on moving more of the flags (ports) behind abstractions borrowed from CRI.
I'd like to isolate all knowledge of docker / podman style flags to pkg/container/... by the next milestone, at which point we should be able to do podman support properly with it's own implementation of all functionality.