Fix /proc/drbd parsing in presence of gaps

In case there are gaps in /proc/drbd related to the minors sequence,
this can lead to empty lines (see the attached data file). In this case,
we currently fail to parse the file correctly.

The patch fixes this by skipping empty lines in
bdev.BaseDRBD._MassageProcData and adds a data file and unittest for
this condition.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>