OCI compatibility layer to make Buildah work with ch-run instead of runc

Write and document an OCI compatibility layer for ch-run sufficient to make buildah bud and buildah push in test/Build.buildah work with it.

Tests including Build.buildah should pass in both Travis (Ubuntu) and the CentOS 7 VM image.

One useful resource from @rstyd: https://github.com/opencontainers/runtime-spec/blob/master/runtime.md