burnin: fix default growth parameter
burnin: fix default growth parameter

Reviewed-by: ultrotter

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