Deploy functions with node_modules included

```
Tools: 5.1.1
Platform: Ubuntu
```

### Steps to reproduce

I was upgrading the `firebase-tools` from `3.17.7` to the latest version `5.1.1` since our CI was unable to deploy the functions due to the following error:

```
HTTP Error: 410, This version of the Firebase CLI is no longer able to deploy to Firebase. Please upgrade to a newer version (>= 4.1.0). If you have further questions, please reach out to Firebase support.
```

After upgrading to the latest `firebase-tools` and from `node 6` to `node 8`, functions still fail to deploy. It appears to me that now `npm install` is executed before finally deploying the new functions. This is problematic since one of your dependency is a private git repository and we use `yarn` instead of `npm`

We would like to pre-package the `node_modules` directory and skip the `npm install`. I checked the package after running `firebase deploy` and it includes the `node_modules` directory. I guess since we have some ignore rules in our `firebase.json`

Is it possible to upload the fully built package to firebase without running `npm install`? I checked the documentation about [handling dependencies](https://cloud.google.com/functions/docs/concepts/nodejs-8-runtime#specifying_dependencies) and it seems that it's possible using `glcoud`, though without further description on how to achieve this.