Add support for flexible environment variable settings

[yorinasub17]

e.g being able to specify:

    spec:
      containers:
      - image: foobar:latest
        name: go-test-monitor-container
        imagePullPolicy: Always
        env:
          - name: DD_AGENT_HOST
            valueFrom:
              fieldRef:
                fieldPath: status.hostIP
          - name: DD_ENTITY_ID
            valueFrom:
              fieldRef:
                fieldPath: metadata.uid 

[yorinasub17]

Suggested implementation is to expose an input value called additional_container_env which can be used to pass in raw list items that are rendered for the env map.

[jjneely]

Implementation: #60

[yorinasub17]

This is now released as v0.1.1