Add test for version dispatching code

Based on the version of DRBD (as reported through /proc/drbd), the DRBD8
class chooses helper objects which adapt for the differences between
DRBD <= 8.3 and DRBD 8.4. This new test verifies that the proper objects
are created.

Signed-off-by: Thomas Thrainer <thomasth@google.com>
Signed-off-by: Michele Tartara <mtartara@google.com>
Reviewed-by: Michele Tartara <mtartara@google.com>