Rename DRBD8 to DRBD8Dev

Right now the DRBD8 class has multiple responsibilities: a) it
reprensents a device which can be set up, grown, etc. and b) it
represents the whole DRBD system on a node which has a usermode helper,
which knows how to shut down _all_ devices, etc. Therefore, the DRBD8Dev
class will only be responsible for a device, whereas the soon to be
extraced DRBD8 class is responsible for the DRBD system.

Signed-off-by: Thomas Thrainer <thomasth@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>