Allow option to return only API-available locations when running getLocations

[tkadlec]

Right now, running getLocations results in all locations being returned. Not all of those locations are actually available to interact with via the API, however.

Hitting the getLocations url directly, we have the ability to filter out non-API locations by adding a querystring. For example, this URL returns all locations, regardless of API availability:

https://webpagetest.org/getLocations.php

Whereas this URL returns only API available locations:

https://webpagetest.org/getLocations.php?k=A

Arguably, that should be the default for the node wrapper, but at the very least, we should provide an option for folks to filter out locations they can't run tests on.