Sync with apache/spark/master

[liyinan926]

Notable changes in apache/spark/master post 2.3.0:

• New config key for specifying physical executor cpu requests: [SPARK-23285][K8S] Allow fractional values for spark.executor.cores apache/spark#20460 (covered in Added support for new configs for specifying image-pull secrets, executor core requests, and secretKeyRefs #158).
• Removal of the init-container: [SPARK-22839][K8S] Remove the use of init-container for downloading remote dependencies apache/spark#20669. Note that this PR also introduced a ConfigMap carrying Spark configuration properties in a file for the driver. The environment variable SPARK_CONF_DIR is set to point to the mount path /opt/spark/conf of the ConfigMap. This is in conflict with what spec.sparkConfigMap is designed to do. Created Deal with internally created ConfigMap mounted onto SPARK_CONF_DIR #216.
• New config key for specifying image pull secrets: [SPARK-23668][K8S] Add config option for passing through k8s Pod.spec.imagePullSecrets apache/spark#20811 (covered in Added support for new configs for specifying image-pull secrets, executor core requests, and secretKeyRefs #158).
• Refactoring (internal and won't affect the operator): [SPARK-22839] [K8s] Refactor to unify driver and executor pod builder APIs apache/spark#20910.
• Memory request change (won't affect the operator): [SPARK-23825][K8s] Requesting memory + memory overhead for pod memory apache/spark#20943.
• PySpark support: [SPARK-23984][K8S] Initial Python Bindings for PySpark on K8s apache/spark#21092 (the operator should work for PySpark out-of-the-box similarly to what branch spark-2.2-support is) (covered in Make the operator work for PySpark in spark master #181).
• New config mechanism for specifying and mounting volumes: [SPARK-23529][K8s] Support mounting volumes apache/spark#21260 (won't use this as it currently only supports hostPath, emptyDir, and PVC).
• New config key for specifying secretKeyRef: [SPARK-24232][k8s] Add support for secret env vars apache/spark#21317 (covered in Added support for new configs for specifying image-pull secrets, executor core requests, and secretKeyRefs #158).