Kubernetes secrets are Base64-encoded

[gsson]

According to the Kubernetes API docs:

Data contains the secret data. Each key must consist of alphanumeric characters, '-', '_' or '.'. The serialized form of the secret data is a base64 encoded string, representing the arbitrary (possibly non-string) data value here. Described in https://tools.ietf.org/html/rfc4648#section-4

This patch decodes keys using Base64 before returning them. Tests have been updated and I've successfully ran it against our v1.7 kube cluster.

Looking at the result, I would like some input on:

• In ConfigMapStore.get(): Normally I wouldn't add if-statements to check for secret like that, but that seems to be the pattern in use. Probably time to refactor something...
• In ConfigMapStore.asSecretObjectMap(): The back-and-forth to Strings seem a bit excessive

[cescoffier]

At some point, we may provide a configmap implementation and a secret implementation. For right now the if-then-else is the adequate approach.