Allow a secret to exist on multiple namespaces

[digiserg]

The change would allow the same secret to being synced to multiple namespaces. This can be very useful for secrets such as private docker registries. The new schema could look like the example below:

---
apiVersion: digitalis.io/v1
kind: ValsSecret
metadata:
  name: private-registry
spec:
  type: kubernetes.io/dockerconfigjson
  namespaces:
    - one
    - two
    - three
  data:
    auth:
      ref: "ref+vault://secret/registry/dockerconfigjson"
      encoding: text