This is a followup to the investigation at #3327, splitting out a piece of #2541

It looks like it should be possible to start supporting runc at the app-level without too many problems. As such, as part of the OCI evolution we should start preparing for supporting runtime-spec at the app level and land some initial code for that.

Pending tasks:

• investigate semantic mapping appc->runtime-spec
• start translating CLI and pod-manifest runtime options into a runtime-spec rendered in stage1
• if we start accepting external JSON config (eg. from k8s), investigate how to bridge information back to the pod manifest

/cc @squeed