Add optional changeId parameter to com.mirantis.mk.Git.updateChangeRequest function

Change-Id: I5da5b77080d30d8504364dde7ce054efffe5d489
Related-PROD: PRODX-3125
diff --git a/src/com/mirantis/mk/Git.groovy b/src/com/mirantis/mk/Git.groovy
index d724b8f..5108cda 100644
--- a/src/com/mirantis/mk/Git.groovy
+++ b/src/com/mirantis/mk/Git.groovy
@@ -529,7 +529,7 @@
 
     def changeParams = ['owner': auth['USER'], 'status': status, 'project': project, 'branch': branch, 'topic': topic]
     def gerritChange = gerrit.findGerritChange(creds, auth, changeParams)
-    def changeId
+    def changeId = params.get('changeId', '')
     def commit
     if (gerritChange) {
         def jsonChange = readJSON text: gerritChange