Expose raw ports using docker2aci style port syntax

[dmwilcox]

This change was created to allow ports to be exposed without being in the image manifest. It uses the same syntax as generated manifests from docker2aci (e.g. '80-tcp', etc). Motivation is for running Envoy as a sidecar -- which discovers the ports it listens on at run-time rather than at image build time.

Happy to add tests + docs or change strategy if desired. I wanted to use OCI syntax as suggested in #2113 but this requires a change to ACName. The advantage of that approach is not overloading the docker syntax -- wherein people may get confused that a docker image port has changed and they no longer get an error but instead have exposed a raw port. I'm happy to pursue that route as well.

I'm not opposed to other strategies such as #3407 but am concerned it appeared to have died on the vine. Rkt's native pod support is an excellent feature for doing sidecars right at the runtime layer and I really want to see service mesh sidecars like Envoy compatible with Rkt.

[SuperQ]

/cc @squeed

[dmwilcox]

Hi @lucab I see you've looked at a few PRs trying to add raw ports to rkt before -- do you have some time to look at this with me? Thank you!