Support port-forward cmd with podman runtime

[hidalgopl]

Currently /rest/v1/portforward endpoint will return server error while running itzo with podman as container runtime.

To run itzo with podman, you should run in GCE and add this to your provider config:

bootImageSpec:
    project: "elotl-kip"
    image: "elotl-kipdev-podman"

and

cellConfig:
    itzoFlag-use-podman: true

you can also write e2e tests and run them locally - check server_podman_test.go

I suggest to add PortForward() method to runtime/runtime.go RuntimeService interface, and refactor servePortForward handler to call s.podCtl.runtime.PortForward(). This would probably require moving current exec implementation for itzo behind this new interface method.

bonus points for making servePortForward cleaner

[hidalgopl]

looks like vk has no support for port-forward: virtual-kubelet/virtual-kubelet#170 so closing this one