Fix cluster rename
Fix cluster rename

With the recent configwriter/ssconf changes, cluster rename becomes
trivial. This patch gets rids of the code and just updates the cluster
object.

Reviewed-by: imsnah

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