Avoid Credentials-sharing behavior of new JobConf(JobConf).

The JobConf(JobConf) constructor causes derived JobConfs to share Credentials. In hadoop 2.3, at least, during job submission the Credentials are mutated (a shuffle token is added). This happens in separate threads on the same underlying Credentials instance, which is not thread-safe and so the instance can become corrupted.

This patch works around the problem by tricking the JobConf constructor into not copying the Credentials reference (by passing it a Configuration that is not a JobConf) and then manually copying the Credentials.