Update stage 3 base image

While the base image can be set via the Jenkins job parameters,
the code to use that parameter was not present in the v10 branch.
This change uses the OPENSTACK_BASE_IMAGE parameter

Change-Id: I229cdca38e605b419a7528031fec542049311c48
Signed-off-by: beierlm <mark.beierl@canonical.com>
diff --git a/jenkins/ci-pipelines/ci_stage_3.groovy b/jenkins/ci-pipelines/ci_stage_3.groovy
index 1542cd7..8b43992 100644
--- a/jenkins/ci-pipelines/ci_stage_3.groovy
+++ b/jenkins/ci-pipelines/ci_stage_3.groovy
@@ -361,7 +361,7 @@
                     output=sh(returnStdout: true, script: """#!/bin/sh -e
                         for line in `grep OS ~/hive/robot-systest.cfg | grep -v OS_CLOUD` ; do export \$line ; done
                         openstack server create --flavor osm.sanity \
-                                                --image ubuntu18.04 \
+                                                --image ${OPENSTACK_BASE_IMAGE} \
                                                 --key-name CICD \
                                                 --property build_url="${BUILD_URL}" \
                                                 --nic net-id=osm-ext \