Support CVMFS based image type in unified containerizer.

CERN developed and is using CernVM File System (CVMFS) to distribute terabytes of software to hundreds of thousands of nodes across hundreds of datacenters. CVMFS uses a combination of lazy loading, extensive indexing, deduplication, caching and geographic distribution. This help to significantly reduce the network load and speed-up the distribution.

All these features make CVMFS a very attractive candidate for solving container-image management and distribution problem in Mesos. To get more context, please refer to the MesosCon'16 talk.

To integrate with Unified Containerizer, we can add a new Image type (i.e., CVMFS) and a new Provisioner Store. We should always use the bind backend so that we can leverage the on-demand fetching feature of CVMFS.

The prototype implementation can be found here:
https://github.com/jieyu/mesos/tree/cvmfs_demo