Add flinkPropertiesSecret for supporting secret properties

[devholic]

Background

Add possibilities for mounting secret properties from Secret.

Resolves #383

Implementation strategy

As @shashken described at https://github.com/GoogleCloudPlatform/flink-on-k8s-operator/issues/383#issuecomment-747453395,

Use flinkPropertiesSecret option in CRD and mount the entire properties into a Secret like @functicons suggested.

• If flinkPropertiesSecret set,
  • Create the config directory to be an emptyDir with a memory medium.
  • Create an init container that copies config files in ConfigMap to config directory since mounting specific file from emptyDir is not possible.
  • Write secrets to /opt/flink/conf/flink-conf.yaml
  • Config directory will be readonly mounted to main containers as same as current ConfigMap implementation.
• Or not, mount ConfigMap as config volume like as-is.

Limitations

• Config keys can be duplicated
• As @chrispatmore mentioned (https://github.com/GoogleCloudPlatform/flink-on-k8s-operator/issues/383#issuecomment-747643831), we cannot react to ConfigMap updates without restarting the pod