feat: support multiple useas modes

[srikumar003]

Fixes #271

BY merging this PR, the following pod definition

apiVersion: v1
kind: Pod
metadata:
  name: busybox
  labels:
    app: busybox
    dataset.0.id: "tests3a"
    dataset.0.useas: "mount.configmap"

will lead to a pod definition like below:

apiVersion: v1
kind: Pod
metadata:
...
spec:
  containers:
...
    envFrom:
    - configMapRef:
        name: tests3a
      prefix: tests3a_
    - prefix: tests3a_
      secretRef:
        name: tests3a
...
    volumeMounts:
...
    - mountPath: /mnt/datasets/tests3a
      name: tests3a
...
  volumes:
...
  - name: tests3a
    persistentVolumeClaim:
      claimName: tests3a