Only update ssconf on cluster serial change
Only update ssconf on cluster serial change

There is no need to update ssconf if the cluster serial number has not
changed.

Reviewed-by: ultrotter

ganeti-github-testing/ganeti-test-1,onponomarev/ganeti,apyrgio/snf-ganeti,yiannist/ganeti,leshchevds/ganeti,bitemyapp/ganeti,dimara/ganeti,ganeti/ganeti,onponomarev/ganeti,ganeti/ganeti,ganeti-github-testing/ganeti-test-1,mbakke/ganeti,bitemyapp/ganeti,ganeti/ganeti,andir/ganeti,leshchevds/ganeti,grnet/snf-ganeti,grnet/snf-ganeti,dimara/ganeti,yiannist/ganeti,andir/ganeti,apyrgio/ganeti,yiannist/ganeti,mbakke/ganeti,apyrgio/snf-ganeti,andir/ganeti,mbakke/ganeti,leshchevds/ganeti,apyrgio/ganeti