de-parallelized ci-tests (#65)

## Description

De-parallelized CI tests (Jenkins)

Author: backstreetkiwi

.ci/integration-tests/hcloud-centos-8/test.sh

@@ -1,6 +1,6 @@
 #!/bin/bash
 git clone -b "$GIT_BRANCH" https://github.com/stackabletech/spark-k8s-operator.git
-(cd spark-k8s-operator/ && ./scripts/run_tests.sh)
+(cd spark-k8s-operator/ && ./scripts/run_tests.sh --parallel 1)
 exit_code=$?
 ./operator-logs.sh spark-k8s > /target/spark-k8s-operator.log
 ./operator-logs.sh spark > /target/spark-operator.log