Implement a File-system pre-population feature

[vschiavoni]

Provide each of the selected splayds for a given job with some content already available
in their virtual file-system.
At first, the system should allow the submission of some content (single files or zipped content).
Users can submit/list/delete contents to/from the controller (splay-list-content).
When submitting a job, users can:

1. specify the content_id(s) to be included in the virtualfs: in this case, a copy of the file is shipped to ALL the selected splayds
2. specify a content_id_map, a simple text file where it is specified to which splayd send which content_id. Example:

  13 abc
  123 def
  200 ghi,lmn

This file says that:

• the file whose content_id is 'abc' must be shipped to splayd 13
• content_id 'def' is for 123.
• splayd 200 must receive two files, whose content_ids are 'ghi' and 'lmn'.

Only these two splayds should then receive their own files