Export the data collected by DRBD in JSON format

Write an intermediate function computing the JSON data before adding them
to the DCReport.

Directly export the data computed by that function (with default parameters)
allowing them to be used (in the future) by the monitoring agent.

Signed-off-by: Michele Tartara <mtartara@google.com>
Reviewed-by: Bernardo Dal Seno <bdalseno@google.com>