RM to try to auth with HDFS on startup, retry with max diagnostics on failure

If the RM can't auth with HDFS, this can first surface during job submission, which can cause confusion about what's wrong and whose credentials are playing up.

Instead, the RM could try to talk to HDFS on launch, ls / should suffice. If it can't auth, it can then tell UGI to log more and retry.

I don't know what the policy should be if the RM can't auth to HDFS at this point. Certainly it can't currently accept work. But should it fail fast or keep going in the hope that the problem is in the KDC or NN and will fix itself without an RM restart?