Add ability to specify source credentials if required

PROD-31387

Change-Id: I6425a37a3f179e13c5e6cb4184855169a0df63cf
(cherry picked from commit dbae4e5621bf4ab3a771247f9789a0a6d4697545)
diff --git a/jenkins/client/job/git-mirrors/downstream/init.yml b/jenkins/client/job/git-mirrors/downstream/init.yml
index 5a6257c..39aaa5f 100644
--- a/jenkins/client/job/git-mirrors/downstream/init.yml
+++ b/jenkins/client/job/git-mirrors/downstream/init.yml
@@ -25,6 +25,9 @@
               SOURCE_URL:
                 type: string
                 default: "{{upstream}}"
+              SOURCE_CREDENTIALS:
+                type: string
+                default: "{{source_credentials}}"
               TARGET_URL:
                 type: string
                 default: "${_param:jenkins_gerrit_url}/{{downstream}}"