Prompt for a specific container in multi-container pods on exec

[ifitzpat]

Hi @abrochard,

This PR adds the ability to choose a specific container when doing an exec (using kubernetes-tramp) into a multi-container pod instead of using the default container. It effectively adds "-c containername" to the kubectl arguments where containername is filled in based on a completing read with a list of available containers. If the pod is a single-container pod the only available container is chosen without the call to completing-read.