Fix cluster verify for empty node groups
Fix cluster verify for empty node groups

There were some implicit assertions in the code that all node groups
have nodes, which is not necessarily true.

Additionally, the patch does a wrapping change.

Signed-off-by: Iustin Pop <acf83a5274f31a43af6b914614365999469c1a31@google.com>
Reviewed-by: René Nussbaumer <bc755ac45a0e799d1d490cba76febc56d80cc5fc@google.com>

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