v0.21.0

This version has no changes at kubernetes-nmstate itself, but there are
some fixes at testing so we mark them with a release.

# Installation

First, install kubernetes-nmstate operator:

```
kubectl apply -f https://github.com/nmstate/kubernetes-nmstate/releases/download/v0.21.0/nmstate.io_nmstates_crd.yaml
kubectl apply -f https://github.com/nmstate/kubernetes-nmstate/releases/download/v0.21.0/namespace.yaml
kubectl apply -f https://github.com/nmstate/kubernetes-nmstate/releases/download/v0.21.0/service_account.yaml
kubectl apply -f https://github.com/nmstate/kubernetes-nmstate/releases/download/v0.21.0/role.yaml
kubectl apply -f https://github.com/nmstate/kubernetes-nmstate/releases/download/v0.21.0/role_binding.yaml
kubectl apply -f https://github.com/nmstate/kubernetes-nmstate/releases/download/v0.21.0/operator.yaml
```

Once that's done, create an `NMState` CR, triggering deployment of
kubernetes-nmstate handler:

```
cat <<EOF | kubectl create -f -
apiVersion: nmstate.io/v1beta1
kind: NMState
metadata:
  name: nmstate
EOF
```
