Support exec cmd with podman runtime

[hidalgopl]

Currently /rest/v1/exec 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 Exec() method to runtime/runtime.go RuntimeService interface, and refactor serveExec handler to call s.podCtl.runtime.Exec(). This would probably require moving current exec implementation for itzo behind this new interface method.