Add basic support for disk parameters

Add basic support for disk parameters

objects.py:
  * add disk parameters to Disk, Cluster, NodeGroup.

constants.py:
  * add dictionaries that will hold types and default values for disk
    parameters (for now, empty).

test/ganeti.constants_unittest.py:
  * add unit tests for consistency in disk parameters default values.

rest of files:
  * add to gnt-cluster and gnt-group the options to manipulate disk
    parameters.

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