Support instance-minor pairing in the DRBD collector

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>