ReadOnlyRootFilesystem support

[burmanm]

What this PR does:
Adds new Spec field: ReadOnlyRootFilesystem that adds necessary new volumes and disables MCAC (since it can't work in this system).

See required PR to get it working (smoke_test_read_only_fs uses image based on this PR): k8ssandra/management-api-for-apache-cassandra#491

Which issue(s) this PR fixes:
Fixes #196
Fixes #199
Fixes #210

Checklist

• Changes manually tested
• Automated Tests added/updated
• Documentation added/updated
• CHANGELOG.md updated (not required for documentation PRs)
• CLA Signed: DataStax CLA

[burmanm]

This PR requires modified management-api to start. Here's a YAML to test..

apiVersion: cassandra.datastax.com/v1beta1
kind: CassandraDatacenter
metadata:
  name: dc1
  annotations:
    cassandra.datastax.com/readonly-fs: "true"
spec:
  clusterName: cluster1
  serverType: cassandra
  serverVersion: "4.1.4"
  serverImage: michaelburman290/cass-management-api:4.1.4
  managementApiAuth:
    insecure: {}
  size: 2
  storageConfig:
      cassandraDataVolumeClaimSpec:
        storageClassName: standard
        accessModes:
          - ReadWriteOnce
        resources:
          requests:
            storage: 1Gi
  racks:
    - name: r1
    - name: r2
  podTemplateSpec:
    spec:
      containers:
        - name: "cassandra"
          securityContext:
            readOnlyRootFilesystem: true
      securityContext:
        runAsUser: 12345
        runAsGroup: 0
        runAsNonRoot: true
  config:
    jvm-server-options:
      initial_heap_size: "512m"
      max_heap_size: "512m"
      additional-jvm-opts:
        - "-Dcassandra.system_distributed_replication_per_dc=1"

[adejanovski]

Works as expected 👍