Add default labels and annotations from spec.json

[captncraig]

Implements #365

This allows you to have in your spec.json:

"spec": {
  "resourceDefaults": {
    "labels": {"foo":"bar"},
    "annotations": {"whatever/foo": "anything_at_all"},
  }
}

and those will be applied to EVERY resource in the environment.

It will not overwrite any labels or annotations explicitly defined in your manifests.

[sh0rez]

Suggested change

	// ResourceDefaults should be inserted in any manifests that tanka processes.
	// ResourceDefaults is inserted in any manifests that tanka processes.

[sh0rez]

Suggested change

	func ApplyDefaults(list manifest.List, cfg v1alpha1.Config) manifest.List {
	func ResourceDefaults(list manifest.List, cfg v1alpha1.Config) manifest.List {