Use labels instead of pod name for pod log read in k8s exec

[dstandish]

This means we don't have to use ti.hostname as a proxy for pod name, and allows us to lift the 63 charcter limit, which was a consequence of getting pod name through hostname.

Using @uranusjr-style "protected" methods just, as always, to contain "public" surface area

[ashb]

Instead of raising an error (which I think will result in a 500!) should we do something like we do on L191-192:

                log += f"*** {str(e)}\n"
                return log, {"end_of_log": True}

[dstandish]

it is already in a try / except so that catches and does just that

[dstandish]

so i could duplicate that code or just raise :)