Check for instances on “gnt-cluster destroy”.
Check for instances on “gnt-cluster destroy”.

Reviewed-by: iustinp


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