Change API to track deployments

* Add get_values() method on Require and Provide to serialize Requires objects
  in a primitive that can be used to fill Requires.
  (list of tuples (variable_xpath, variable_value)).

  Will be used to save the provide calls history and be able to reconstruct Requires
  on agent start from the history (running a check method with last requires
  for example)

* API changes for requires.

  From [(variable_xpath, variable_vaule), ...] we should now send
  [[(variable_xpath, variable_vaule), ...], {'source': xpath, ...}]