fix(kubernetes): let kubectl handle namespaces

[sh0rez]

Instead of naively setting the default namespaces on objects that don't
have one, we now overlay $KUBECONFIG with a patch to set the default
namespace on the context, so that kubectl will do the job for us.

kubectl does this far more intelligent than we did, especially it does
not inject the namespace on objects that don't take one.

Fixes #190

There is one TODO left in injecting the patch into $KUBECONFIG:

tanka/pkg/kubernetes/client/kubectl.go

Lines 68 to 72 in 0257f10

	for i, s := range env {
	// TODO: handle empty $KUBECONFIG
	if strings.HasPrefix(s, "KUBECONFIG=") {
	env[i] = fmt.Sprintf("KUBECONFIG=%s:%s", k.nsPatch, strings.TrimPrefix(s, "KUBECONFIG="))
	}

[rfratto]

Nice, this is a good approach. Just one comment about handling the lack of $KUBECONFIG now rather than having to do it later.

[sh0rez]

Fixed an issue with native always returning a diff for things on the default namespace.

Added tests:

• separating objects in missing namespace and ready during native diff (old functionality, broke here)
• injecting $KUBECONFIG

PTAL @rfratto

[rfratto]

LGTM!