Extend DRBD disks with shared secret attribute

This patch, which is similar to r1679 (Extend DRBD disks with minors
attribute), extends the logical and physical id of the DRBD disks with a
shared secret attribute. This is generated at disk creation time and
saved in the config file.

The generation of the secret is done so that we don't have duplicates in
the configuration (otherwise the goal of preventing cross-connection
will not be reached), so we add to config.py more than just a simple
call to utils.GenerateSecret().

The patch does not yet enable the use of the secrets.

Reviewed-by: imsnah