Add gitlab-ctl command to identify the master node in a Redis HA situation

Related to #3785 (closed). While upgrading an instance with Redis HA, the master node needs to be upgraded last. For this, the user should be able to know which is the current master node. Right now, it is possible only by manually running the /opt/gitlab/embedded/bin/redis-cli command, which is not so easy. We should provide a gitlab-ctl command which does this.