[TACHYON-30] cephfs as under filesystem

This makes cephfs as a new under filesystem.

The unit test environment is created by vagrant, detailed information can be found at vagrant/README.md.

we only use HcfsCluster for cephfs and glusterfs profiles. So mUri should be set to either ceph://localhost:6789/ or glusterfs:///. 

But your reading is also right, the code alone does make mUri == null possible.