ssconf: add some more keys and some fixes

This patch adds the online node list and instance list to the ssconf
keys. In order to do distribute correctly the instance list, we need to
update the cluster serial number on instance additions and removals.

The patch also changes the permissions on the ssconf files to be 0444:
  - no write for root, in order to signal that these file should not be
    modified
  - read for everyone since the files don't contain sensitive data
    anymore (and permissions can be controlled via the parent directory
    if needed)

The patch also fixes a small typo on gnt-cluster.

Reviewed-by: ultrotter