Add the stand-alone data collector for DRBD

It is implemented as a single binary receiving, as its first parameter,
the name of the actual data collector to be run.
This way, its structure can be used for all the future data collectors.

Also, factored out of bdev.py into constants.py the location of the DRBD
status file.

Signed-off-by: Michele Tartara <mtartara@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>