mgr: backward compatibility for module management

mgr: backward compatibility for module management

Follow up on 3abc253fecc91f29c90e23ae95e1b83f8ffd3de6

The structure had even changed within `luminous` release.
It was first:

```
{
    "enabled_modules": [
        "balancer",
        "dashboard",
        "restful",
        "status"
    ],
    "disabled_modules": [
        "influx",
        "localpool",
        "prometheus",
        "selftest",
        "zabbix"
    ]
}
```
Then it changed for:

```
{
  "enabled_modules": [
      "status"
  ],
  "disabled_modules": [
      "balancer",
      "dashboard",
      "influx",
      "localpool",
      "prometheus",
      "restful",
      "selftest",
      "zabbix"
  ]
}
```

and finally:
```
{
  "enabled_modules": [
      "status"
  ],
  "disabled_modules": [
      {
          "name": "balancer",
          "can_run": true,
          "error_string": ""
      },
      {
          "name": "dashboard",
          "can_run": true,
          "error_string": ""
      }
  ]
}
```

Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>