This commits enables the DRBD data collector to use the Confd client to
gather information about the pairing between DRBD minors and instances.

For testing purposes, the DRBD data collector now requires either zero
or two parameters: one is the DRBD file, one is the pairings file.
When no parameter is passed, the collector is in "production mode" and takes
the data from /proc/drbd and from the Confd client.

The shell tests of mon-collector are updated accordingly.

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

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>