Description of problem:
Even when ceph-mgr balancer is enabled, the output of `ceph mgr module ls` does not show it. It should appear as iostat, prometheus, etc do.

bash-4.2# ceph mgr module enable balancer
module 'balancer' is already enabled (always-on)
bash-4.2# ceph mgr module ls |grep -A4 enable
    "enabled_modules": [
        "iostat",
        "prometheus",
        "restful"
    ],

Version-Release number of selected component (if applicable): 4.0