Enable support for virtual hosts

When Wookie is deployed using a virtual host configuration, it still sends instance URLs that use the real server location.

E.g. if Wookie is deployed at "bernard.myco.com:8080/wookie" but is served from the virtual host "services.myco.com/wookie" requests to the REST API to instantiate a widget returns a URL starting "bernard.myco.com:8080".

It should be possible to specify a vhost server name in configuration to override this behaviour.

NB There may be other functions in Wookie that would also need to be aware of any vhost settings.

(Originally suggested by David Sherlock - thanks David!)