Support S3 credentials through a secret

[fhennig]

We want an s3 section in the CRD, for now we can use the same structure as in druid:

s3:
  endpoint: s3-de-central.profitbricks.com
  credentialsSecret: s3-credentials

for the endpoint there is a configoption, the access key and secret key should be mounted from the referenced secret. The secret structure used in druid is:

apiVersion: v1
kind: Secret
metadata:
  name: s3-credentials
stringData:
  accessKeyId: YOURACCESSKEYIDHERE
  secretAccessKey: YOURSECRETACCESSKEYHERE

The secret should be mounted and the env vars for that are AWS_ACCESS_KEY_ID AWS_SECRET_ACCESS_KEY. The secret also needs to be mounted in the executors, so they can read from S3 too; so the pod template needs to be adjusted accordingly.