[SPARK-26083][K8S] Add Copy pyspark into corresponding dir cmd in pyspark Dockerfile

When I try to run `./bin/pyspark` cmd in a pod in Kubernetes(image built without change from pyspark Dockerfile), I'm getting an error:
```
$SPARK_HOME/bin/pyspark --deploy-mode client --master k8s://https://$KUBERNETES_SERVICE_HOST:$KUBERNETES_SERVICE_PORT_HTTPS ...
Python 2.7.15 (default, Aug 22 2018, 13:24:18)
[GCC 6.4.0] on linux2
Type "help", "copyright", "credits" or "license" for more information.
Could not open PYTHONSTARTUP
IOError: [Errno 2] No such file or directory: '/opt/spark/python/pyspark/shell.py'
```
This is because `pyspark` folder doesn't exist under `/opt/spark/python/`

## What changes were proposed in this pull request?

Added `COPY python/pyspark ${SPARK_HOME}/python/pyspark` to pyspark Dockerfile to resolve issue above.

## How was this patch tested?
Google Kubernetes Engine

Closes apache#23037 from AzureQ/master.

Authored-by: Qi Shao <[email protected]>
Signed-off-by: Marcelo Vanzin <[email protected]>

Author: AzureQ

bin/docker-image-tool.sh

@@ -107,6 +107,7 @@ function create_dev_build_context {(
     "$PYSPARK_CTX/kubernetes/dockerfiles"
   mkdir "$PYSPARK_CTX/python"
   cp -r "python/lib" "$PYSPARK_CTX/python/lib"
+  cp -r "python/pyspark" "$PYSPARK_CTX/python/pyspark"
 
   local R_CTX="$CTX_DIR/sparkr"
   mkdir -p "$R_CTX/kubernetes"

resource-managers/kubernetes/docker/src/main/dockerfiles/spark/bindings/python/Dockerfile

@@ -38,6 +38,7 @@ RUN apk add --no-cache python && \
     # Removed the .cache to save space
     rm -r /root/.cache
 
+COPY python/pyspark ${SPARK_HOME}/python/pyspark
 COPY python/lib ${SPARK_HOME}/python/lib
 ENV PYTHONPATH ${SPARK_HOME}/python/lib/pyspark.zip:${SPARK_HOME}/python/lib/py4j-*.zip