Add `drbdsetup show` parser for DRBD 8.4

Common functionality between the DRBD 8.3 and DRBD 8.4 parser has been
extracted into BaseShowInfo. A test which verifies the behaviour is
included, but the DRBD84ShowInfo class is not yet used within the DRBD8
class.

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