dogrc file shouldn't be required if api/app key env vars are set

```
$ DATADOG_API_KEY=foo DATADOG_APP_KEY=bar dog timeboard show_all
~/.dogrc does not exist. Would you like to create it? [Y/n] 
```

Even tho though env vars are used later on in the init https://github.com/DataDog/datadogpy/blob/master/datadog/__init__.py#L63-L64

This makes it difficult to automate things that only have access to env vars and not the file system.
