rotate static AWS IAM credentials

[ribbybibby]

This adds a deployment that allows for frequent and automated rotation of the IAM credentials used by the AWS secret backend.

It consists of:

1. A service that periodically calls /v1/aws/config/rotate-root to rotate the access key
2. A script which is ran by the user when setting up a new cluster that sets the initial credentials

These are the steps that would need to be taken as part of the bootstrapping:

$ aws iam create-access-key --user-name <username>
  <output containing keys>
$ kubectl --context=<context> -n <namespace> exec -it vault-aws-credentials-rotator-<hash> -- vault-aws-credentials.sh init
  Access Key ID: <insert key>
  Secret Access Key: <insert key>

Upsides to this approach include:

• Credentials aren't stored anywhere in git, or known outside of vault at all
• Allows us to adhere to AWS best practices around frequent access key rotation

Downsides:

• Adds some state to the vault cluster. One of the nicest things about our vault setup is that you can completely destroy the namespace and have everything back and working as intended simply by re-applying the manifests. With this change, you would have to bootstrap AWS manually again.
• Adds more complexity to the vault setup in general