gerrit-sync-ve: switch to jenkins-mwext-sync user

Use a new Gerrit user to commit the updates: jenkins-mwext-sync.  This
way we can remove the email address of jenkins-bot user to prevent the
email recipients to be spammed by all Gerrit activities.

Change-Id: Ie1415db5bef8ae9ea2c4b19cf12b82830902b223