qa/suites/rados/singleton-nomsgr/all/balancer: whitelist PG_AVAILABILITY

qa/suites/rados/singleton-nomsgr/all/balancer: whitelist PG_AVAILABILITY

Balancer triggers peering, which may make PGs briefly go inactive--when
they possibly haven't been active yet.  E.g.,

    "PG_AVAILABILITY": {
        "severity": "HEALTH_WARN",
        "summary": {
            "message": "Reduced data availability: 3 pgs inactive, 3 pgs peering",
            "count": 6
        },
        "detail": [
            {
                "message": "pg 2.6 is stuck peering since forever, current state peering, last acting [2,0]"
            },
            {
                "message": "pg 2.1c is stuck peering since forever, current state peering, last acting [2,1]"
            },
            {
                "message": "pg 2.7a is stuck peering since forever, current state peering, last acting [2,0]"
            }
        ]
    }

Signed-off-by: Sage Weil <sage@redhat.com>