(TK-3) Add support for multiple servers

Add support for multiple servers to the jetty9 webservice. Previously, a user would define the webserver key in the configuration file as a map of configuration data, which would be used to configure a single Jetty9 server. This syntax is still valid. However, now the user can also define the webserver key as a nested map, where each key is the id of a server and each value is a map of configuration data (of the same form as before). With this information, the Jetty9 service will now initialize, start-up, and shutdown multiple servers if so specified by the user.
This is a very minor nitpick, but I think I'd prefer that the server-id be the first argument (after `this`) to all of these new `to` functions.  That seems more consistent since the various functions can have differing numbers of arguments after the `server-id`.  Since this is an API design question, though, might be good to get another opinion.