Use disk parameters in noded

Use disk parameters in noded

* add the params attribute to BlockDev, and add the corresponding
  parameter to all the BlockDev classes;
* change the Create, Assemble and FindDevice factory functions interface
  to accept as parameters an objects.Disk instance and a list of
  children block devices; update their callers;
* make the factory functions provide default values for params if
  needed;
* factor out a check in the block device factory functions.

Signed-off-by: Andrea Spadaccini <spadaccio@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>