Add -DskipTests to dev docs (apache-spark-on-k8s#115)

* Add -DskipTests to dev docs

* Remove extraneous skipTests

Author: ash211

resource-managers/kubernetes/README.md

@@ -17,7 +17,7 @@ important matters to keep in mind when developing this feature.
 To build Spark with Kubernetes support, use the `kubernetes` profile when invoking Maven. For example, to simply compile
 the Kubernetes core implementation module along with its dependencies:
 
-    build/mvn compile -Pkubernetes -pl resource-managers/kubernetes/core -am
+    build/mvn compile -Pkubernetes -pl resource-managers/kubernetes/core -am -DskipTests
 
 To build a distribution of Spark with Kubernetes support, use the `dev/make-distribution.sh` script, and add the
 `kubernetes` profile as part of the build arguments. Any other build arguments can be specified as one would expect when
@@ -47,7 +47,7 @@ Running any of the integration tests requires including `kubernetes-integration-
 order to prepare the environment for running the integration tests, the `pre-integration-test` step must be run in Maven
 on the `resource-managers/kubernetes/integration-tests` module:
 
-    build/mvn pre-integration-test -Pkubernetes -Pkubernetes-integration-tests -pl resource-managers/kubernetes/integration-tests -am
+    build/mvn pre-integration-test -Pkubernetes -Pkubernetes-integration-tests -pl resource-managers/kubernetes/integration-tests -am -DskipTests
 
 Afterwards, the integration tests can be executed with Maven or your IDE. Note that when running tests from an IDE, the
 `pre-integration-test` phase must be run every time the Spark main code changes. When running tests from the