This commit adds two extra options:
- enable_cache_disk
- cache_disk_filename

This allows to attach a cache disk image to extend the storage
needs initially provided by the OS image.

In context-based systems such as uCernVM, this image can be
used to store the local cache of artifacs coming from CVMFS.

Note this image is attached without further checking on the content
and is entirely up to the main OS to make good use of it.

Signed-off-by: Carlos Aguado Sanchez <carlos.aguado@epfl.ch>