Add support for native vault/kubernetes auth

[sidewinder12s]

It'd be create if this sidecar could automatically login with the native kubernetes support added to Vault by providing a role_name and pulling the JWT token from a service account.

https://www.vaultproject.io/api/auth/kubernetes/index.html#login

[Zandrael]

hey,

this feature would be great.
first i had started with kubernetes-vault to get an vault-token-file.
in the meantime, i had switched over to the vault auth backend 'kubernetes'.

It is much easier for me to use k8s-service accounts (jwt of them) for vault-authentification. the jwt, which is needed for this method, is being mounted automaticly bye k8s, so, the path is always the same.

some minutes ago, i had implemented this bye using bash:
( since i think, that the steps generally could be the same in golang, i'm attaching my dirt script. Possibly it could save some time)

Sadly im not very into golang yet. otherwise i would make an pull request for this feature, which is highly needed. If ur app could do the kubernetes-authentification, i could reduce the count of init-containers for each deployment.
docker-entrypoint.zip