run integration-cli stages on s390x and ppc64le if not a PR check

Essentially, run on merge to target branch which may or may not be
master branch. Could be 19.03 branch, for example.

See: https://jenkins.io/doc/book/pipeline/syntax/

Signed-off-by: Andrew Hsu <andrewhsu@docker.com>

Jenkinsfile

@@ -463,7 +463,7 @@ pipeline {
                 stage('s390x integration-cli') {
                     when {
                         beforeAgent true
-                        branch 'master'
+                        not { changeRequest() }
                         expression { params.s390x }
                     }
                     agent { label 's390x-ubuntu-1604' }
@@ -647,7 +647,7 @@ pipeline {
                 stage('ppc64le integration-cli') {
                     when {
                         beforeAgent true
-                        branch 'master'
+                        not { changeRequest() }
                         expression { params.ppc64le }
                     }
                     agent { label 'ppc64le-ubuntu-1604' }