TACHYON-198 Move UserConf usage to use TachyonConf

This PR consist next steps to integrate new configuration work:
-) Remove usage of UserConf
-) Push TachyonConf to Tachyon Hadoop's file system (aka TFS class).
-) Remove UserConf.java from repo.
Ah I remember now why I did it that way. 
TachyonFS.get is a public interface which either we change the signature that could make code no longer backward compatible =(
My suggestion is to add another Tachyon.get that takes TachyonConf and keep the "mergeConf" for now to merge the TachyonConf from caller; or expose access to TachyonFS.mTachyonConf as public accessor?