Secrets mount

[sseidman]

Adds a mount secret command that takes as input a string (in the format [{ "secretName": "<secret-name>", "path": "</path/to/secret>" }]) that specifies a list of kubernetes secrets to mount and the path at which to mount them at. Secrets will be written in the the Kubernetes secret format (e.g. filename = username, file = password). Adding this command will allow the k8ssandra-client to be used as the image for k8ssandra/k8ssandra-operator#608.

Verified by loading docker image to a kind cluster and applying the following manifest:

apiVersion: v1
kind: Secret
metadata:
  name: mysecret
type: Opaque
data:
  USER_NAME: YWRtaW4=
  PASSWORD: MWYyZDFlMmU2N2Rm
---
apiVersion: v1
kind: Pod
metadata:
  name: test-inject
  labels:
  annotations:
  namespace: k8ssandra-operator
spec:
  serviceAccountName: k8ssandra-operator
  initContainers:
    - args:
        - mount
        - "[{\"secretName\": \"mysecret\", \"path\": \"/etc/secrets\"}]"
      image: "k8ssandra/k8ssandra-client"
      imagePullPolicy: IfNotPresent
      name: k8ssandra-client
      volumeMounts:
        - mountPath: /etc/secrets
          name: secrets
  containers:
    - args:
        - sleep
        - "3600"
      image: busybox
      name: lifespan-offensive
      volumeMounts:
        - mountPath: /etc/secrets
          name: secrets
  restartPolicy: Always
  volumes:
    - name: secrets
      emptyDir: {}
      ```