Backwards compatibility support "Master" as chief

[jlewi]

For backwards compatibility with older versions of TF.estimator we need to treat replicas with name "Master" as "Chief".

One place where this doesn't seem to be happening is the exit policy. If there is a replica of type, "Master" than TFJob should be marked completed when the "Master" exits. It would be good to add an E2E test for this case.

We should probably add validation to ensure only "Master" or "Chief" is provided.

@ScorpioCPH Would you be willing to pick this up?

[gaocegege]

/assign