Allow setting worker environment variables in kubernetes

[mwylde]

Adds support for setting env vars on the kubernetes workers via config, for example:

configmap.yaml

apiVersion: v1
kind: ConfigMap
metadata:
  name: my-arroyo-config
data:
  config.yaml: |
    kubernetes-scheduler:
      worker:
        env:
          - name: AWS_DEFAULT_REGION
            value: us-east-1
          - name: AWS_ACCESS_KEY_ID
            value: "AJKSDJLAKJSDALKSJD"
          - name: AWS_SECRET_ACCESS_KEY
            value: "214124jHJhkjh1212aasd"

values.yaml

existingConfigMap: "my-arroyo-config"