Add support for dryRun option to kube-conformance image

A common issue users run into is wanting a list of tests
a certain regexp will run, without actually running it.

ginkgo supports this with the dryRun flag but it was not
exposed via the kube-conformance image. This change
will set the flag if the E2E_DRYRUN environment variable
is set.

Fixes kubernetes#74727

Author: johnSchnake

cluster/images/conformance/run_e2e.sh

@@ -45,6 +45,10 @@ ginkgo_args=(
     "--noColor=true"
 )
 
+if [[ -n ${E2E_DRYRUN:-} ]]; then
+    ginkgo_args+=("--dryRun=true")
+fi
+
 case ${E2E_PARALLEL} in
     'y'|'Y')           ginkgo_args+=("--nodes=25") ;;
     [1-9]|[1-9][0-9]*) ginkgo_args+=("--nodes=${E2E_PARALLEL}") ;;