(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 implemented the way I'd expected it to be, but I think that the way some of the other code is written, it'd be possible to define a multi-server config file that didn't have a `:default` key and then get an NPE if you called one of these signatures.  I think `:default` should probably be a required key in the multi-server config format, in order to prevent this; but I think that if you add the additional schema that I suggested above (which does have `:default` as a required key), that will take care of that issue.