Fix cluster start failure due to missing netmask

If the cluster version is upgraded from a version before commit
5a8648eb to a version after that commit,
the master startup will fail because the ssconf file with the master
netmask will not be present, and it is read during the master voting
process.

This patch fixes this bug, making ssconf.GetMasterNetmask return a
default netmask value, and also corrects the docstring of this method.

Signed-off-by: Andrea Spadaccini <spadaccio@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>