fix istioctl cni on gke (#5987)
fix istioctl cni on gke (#5987)

`istioctl manifest apply --set profile=demo --set cni.enabled=true --set values.cni.cniBinDir=/home/kubernetes/bin` by default put the `istio-cni-node` daemonset in istio-system namespace. The cni pod fails to create.

According to the helm command here https://github.com/istio/cni/blob/3fc0e65d94c3d5752d4032d4ccaa8ea462c5cbfe/README.md#usage `istio manifest` should set cni.namespace to kube-system on gke

Signed-off-by: Yuchen Dai <f2550135201283870cb3c3d0a55f74b43525a4a1@gmail.com>
istio/istio.io,istio/istio.io,istio/istio.io,istio/istio.io,istio/istio.io,istio/istio.io,istio/istio.io