add safety to avoid deleting an existing namespace

[qrkourier]

If WEBHOOK_NAMESPACE is an existing namespace, then uninstalling the generated manifest will delete that namespace.

Add safety to the install procedure to ensure that WEBHOOK_NAMESPACE is always a new namespace, or avoid declaring the namespace in ziti-webhook-spec.yaml when WEBHOOK_NAMESPACE happens to exist already.

A simple way to accomplish this is to avoid declaring the Namespace resource in ziti-webhook-spec.yaml. That way, the admin must create the namespace if it does not exist, and uninstalling ziti-webhook-spec.yaml will never put any namespace at risk.

[dariuszSki]

the deployment template https://github.com/netfoundry/ziti-k8s-agent/blob/293b09546224333a76f21cb1db40dca6d6f93ce0/deployment/ziti-webhook-spec.yaml is just an example that users can use.

[dariuszSki]

We created it for the demo, so users can try it out and find out what is needed to run the injector.

[qrkourier]

In my branch, I changed invalid kubectl -f commands like kubectl create -f, which will always fail if the namespace exists. That way, the user must opt-in to deploying in an existing namespace by changing create to apply, which runs the risk of deleting that namespace if they later command kubectl delete -f