Support references into secrets and config maps

[wryun]

This is a step in allowing us to change the 'shape' of secrets, which is particularly important due to the way parametersFrom works in Service Catalog:

https://github.com/kubernetes-incubator/service-catalog/blob/master/docs/parameters.md

(i.e. the secret ends up merged directly into the inputs without the ability to customise the field name)

In the longer term, this functionality would allow:

• plugins to explicitly declare secret dependencies
• objects (e.g. service instances) to directly have non-secret parameters from secrets
• new secrets be created in smith from the outputs of other secrets (however, at the moment this is impossible due to the limited templating language used)