Update branch variator for gerrit refs in generate model job

Change-Id: I7ed7ae68e403941e70a592dd140978731c759105
Related-Prod: PROD-28865
diff --git a/generate-cookiecutter-products.groovy b/generate-cookiecutter-products.groovy
index 50c38b2..63d7b52 100644
--- a/generate-cookiecutter-products.groovy
+++ b/generate-cookiecutter-products.groovy
@@ -83,7 +83,7 @@
     }
 
     [ 'cookiecutter_template_branch', 'shared_reclass_branch', 'mcp_common_scripts_branch' ].each { repoName ->
-        if (context['mcp_version'] in [ "nightly", "testing", "stable" ]) {
+        if (context['mcp_version'] in [ "nightly", "testing", "stable" ] && ! context.get(repoName)) {
             context[repoName] = 'master'
         } else if (! context.get(repoName)) {
             context[repoName] = gitGuessedVersion ?: "release/${context['mcp_version']}".toString()