Set environment variables from secretRef/configMap

[chrbrnracn]

Current situation:

• existing secrets or configMaps can be added via .Values.existingSecret or .Values.existingConfigMap
• only one of each is possible at a time
• it is not possible to set a custom environment variable where the value is set

With the change it is possible to set a named environment variable to a specific key in a specific secret or config map.

Use case:
When running Kafka with Strimzi, each KafkaUser entity gets its own secret with the password and sasl.jaas.config. With the new feature it's possible to directly access this secret and use it for Kafka UI.

  secretMappings:
    KAFKA_CLUSTERS_0_PROPERTIES_SASL_JAAS_CONFIG:
      secretName: mySuperUser
      secretKey: sasl.jaas.config

There may be a lot more use cases for this feature. That's why I added the same for configMapKeyRef.

Side note:
I had to set all other kafka.clusters.* values as envs.config instead of using yamlApplicationConfig, but not auth.* or management.*. It looks like the application does not deep-merge values from config file and YAML configuration. I didn't find this in the docs.

[Haarolean]

@chrbrnracn thanks for your contribution!