make generic HCFS as under filesystem; setup VM environment for under filesystem unit tests

This make generic HCFS as under filesystem.

The unit test environment for Cephfs and Glusterfs is created by vagrant, detailed information can be found at vagrant/{ceph,glusterfs}/README.md.

also, since we already have a docker environment. should we also add virtualbox and vagrant ?
https://github.com/amplab/tachyon/tree/master/docker

@calvinjia @dcapwell @uronce-cc