Use DRBD84ShowInfo based on DRBD version

Now the DRBD8 class actually checks which version of DRBD is running on
the system and chooses the right class to parse the `drbdsetup show`
output.

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