Add unittest for DRBD8 drdbsetup show parser

This patch changes the bdev.DRBD8._GetDevInfo to take a string instead
of a minor, separates the `drbdsetup show` invocation into a new
separate method (bdev.DRBD8._GetShowData) and modifies the rest of the
DRBD8 class to make the appropriate calls.

It also adds a unittest script and data files for testing various cases
of device output.

Reviewed-by: imsnah