Make keyfilename authentication more consistent with other Node libraries

This issue is related to our work in client extensible options.

Other Node libraries are able to authenticate using just a keyfileName in the constructor of the client. However, it is required to add a project_id variable in the constructor to authenticate, like so:

new BigQuery({
    keyFilename: //keyfile
    projectId: 
      process.env.GOOGLE_CLOUD_PROJECT  
  });

In order to be more consistent across libraries/requirements, it would be nice if we could get the project id from the keyfile itself, without having to declare it separately.

I think this work might be similar to this PR: https://github.com/googleapis/nodejs-pubsub/pull/731

[bcoe]

@stephenplusplus @sofisl what’s weird, is I think you were noticing the project is appropriately detected in our auto generated libraries? which share the auth library.

@sofisl when we eventually dig into this, we might want to use npm link to test bigquery and the auth library at the same time.

[sofisl]

Closing in lieu of #1084 in google-auth-library-nodejs.