TACHYON-237: Integrate Tachyon with secured HDFS.

Take a look at https://github.com/amplab/tachyon/blob/master/core/src/main/java/tachyon/Constants.java

This is where a ll String constants should be.

The actual configuration is a static in each *Conf class but the name should be in Constants class.

I am doing major config rework so I appreciate if you could move the name of the config itself into Constants class.