Allow an API endpoint to be explicitly passed

[lukesneeringer]

We currently hardcode api-name.googleapis.com in the Node.js API clients. We should take an options.apiEndpoint that allows the user to override this. This is useful for API producers wanting to use staging servers, customers wanting to test with emulators, etc.

See googleapis/google-cloud-node#1630 (comment)

[evaogbe]

@lukesneeringer Can you add a priority to this?

[evaogbe]

Isn't this already covered in the serviceAddress option?

[evaogbe]

@lukesneeringer @alexander-fenster Is this still relevant?

[alexander-fenster]

We use an option from an annotation, and we let the user override it in the client constructor.