Support Auth Method: Kubernetes

[roechi]

In some use-cases it might be very convenient to authenticate towards vault via a Kubernetes namespace.
Any Kubernetes Pod has access to an account JSON Web Token (JWT). The token is issued for a service account, if no service account is given, a default account will be created. Anyhow, the token can be found at a specific mount point:
/var/run/secrets/kubernetes.io/serviceaccount/token

The Vault Kubernetes Auth plugin allows it, to configure access for specific accounts and namespaces. We can leverage this, to exchange the Service Account JWT for a Vault Access Token. See: https://www.vaultproject.io/docs/auth/kubernetes

I guess this is obviously a great addition for the aws-signing-proxy Docker image but it might also be useful for the vault-env-cred-provider.

[theurichde]

fixed with #52