Implement a server endpoint to return configuration data for the Dashboard

[dbutenhof]

The choice of the dashboard's "prefix" and index (run_index and result_index) configuration variables are tied to a specific server. It would be nice if the server would offer an end-point which would return the configuration relative to that server instead of having endpoints.js built into the javascript bundle loaded on the server.

That way we can build one dashboard bundle and test it on staging and move it to production without having to change or rebuild the bundle.

The dashboard would then need only to be configured to a specific Pbench server host/port via an environment variable, and would automatically configure the rest using the server.

Related to Dashboard issue #107