Add the ability to add Environment variables as a configmap

[kharouny]

Statefulsets have the option to add Environment variables as configmaps like so...

apiVersion: apps/v1
kind: StatefulSet
metadata:
  name: my-statefulset
spec:
  replicas: 1
  template:
    spec:
      containers:
      - name: my-app-container
        image: your-image:tag
        envFrom:
        - configMapRef:
            name: my-app-config

Currently the SolrCloud config only supports adding additional EnvVariables in line.

[HoustonPutman]

Sounds good to me, it would likely be a pretty good first PR if you are interested in making a contribution!