Extract DRBD info parsing into drbd_info.py

Parsing the status (/proc/drbd) and `drbdsetup show` output takes quite
a bit of code, which has to be augmented by even more with the DRBD 8.4
support. So extract all the related classes into their own file.

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