(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.
Same comments as above; better to push some of this into `core` if possible, and use the stricter schemas that I described above.  Might want to create separate functions for the two branches of your `if` statement (or `cond` statement if it ends up being that), to make it a bit more readable.