Add the dynamic-resync DRBD disk parameter

constants.py, cmdlib.py:
- add the dynamic-resync parameter, both at DT and LD levels;

lib/bdev.py:
- change SetSyncSpeed to SetSyncParams, and _SetMinorSyncSpeed to
  _SetMinorSyncParams;
- use the dynamic-resync parameter.

Later patches will implement the rest of the parameters.

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