Initialize and update the serial_no on objects
Initialize and update the serial_no on objects

This patch add initialization of the serial_no on instance and nodes,
and update of the field whenever an object is updated in the generic
case, via ConfigWriter.Update(obj) and in the specific case of
instances' state being modified manually.

Reviewed-by: ultrotter

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