fix crash in `pm2 desc id`

When process start from json file and not declared the 'exec_interpreter' property, `pm2 desc id` command will crash in line [L52](https://github.com/Unitech/pm2/blob/master/lib/CliUx.js#L52).

So I modified the `common.resolveAppPaths` method to add the default 'exec_interpreter' property. It will also set a ext based interpreter property, as the same of `CLI.startFile`.