Issue title: Reload with an ecosystem config, parallel param invalid
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## What's going wrong?

When I reload a [app name] with `parallel`, it works like a charm. But if I put app to an ecosystem.config.json, the `parallel` param seems don't work.

## How could we reproduce this issue?

```js
// ecosystem.config.json
{
  "apps": [
    {
      "name": "master",
      "script": ".index.js",
      "instances": "12",
      "exec_mode": "cluster",
      "wait_ready": true,
      "kill_timeout": 7200000
    }
  ]
}
```

First, start a app using `pm2 start ecosystem.config.json`.
Second, reload it using `pm2 reload ecosystem.config.json --parallel=12`

It will reload 2 processes each time, other than reload all processes once.

## Supporting information

<!--
Please run the following command (available on PM2 >= 2.6)
-->
```
$ pm2 report
```

 Comments: 
 Comment 0: This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

{'source': 'GithubIssues', 'repo_name_with_owner': 'Unitech/pm2', 'issue_id': 615116621, 'repo_stars': 37993.0}