manage the vfs database part into abstraction and mongodb implementation

- VFSFile refer on vpath that is transformed into fpath by the driver
- VFSManager permit the creation of the VFSFile that will hide all the work on
storage and index driver

Change-Id: I8dac2d1ea4a6bcfee6af046e07bcb9187139b1e3
Merge pull request #2 in CHSFRMWRK/chaosframework from features_vfs to development

* commit '1592eea240008ecc5b3e2ec90b1d0a4dc83b2f72':
  manage the vfs database part into abstraction and mongodb implementation