Don't allow renaming to an existing instance
Don't allow renaming to an existing instance

Even if the target instance is down or we are not checking for IP conflicts
changing an instance name to a new one which is already in the cluster is
doomed to fail, because in a lot of places (among which figures the mind of
most users/admins) instance names are assumed to be unique.

Reviewed-by: imsnah


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