[SPARK-24963][K8S][TESTS] Add user-specified service account name for client mode test driver pod

[mccheah]

What changes were proposed in this pull request?

Adds the user-set service account name for the driver pod in the client mode integration test

How was this patch tested?

Manual test against a custom Kubernetes cluster

[mccheah]

@skonto can you sign off here before I merge?

[mccheah]

ok to test

[SparkQA]

Kubernetes integration test starting
URL: https://amplab.cs.berkeley.edu/jenkins/job/testing-k8s-prb-make-spark-distribution-unified/1502/

[SparkQA]

Test build #93808 has finished for PR 21924 at commit 8afc4e9.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Kubernetes integration test status success
URL: https://amplab.cs.berkeley.edu/jenkins/job/testing-k8s-prb-make-spark-distribution-unified/1502/

[skonto]

LGTM. I have the same change locally and it works.
I test like this:

./dev/dev-run-integration-tests.sh --service-account spark-sa --namespace spark --image-tag $SPARK_K8S_IMAGE_TAG --spark-tgz $TGZ_PATH --image-repo $DOCKER_USERNAME