Fix plugin webroots not working under WSGI deployment
Fix plugin webroots not working under WSGI deployment

Prior to this, we were exposing the primary webroot as the WSGI
application. In the case of plugin webroots we mount multiple webroots
onto cherrypy.tree which itself is a WSGI-compliant application.

data-exp-lab/girder,kotfic/girder,data-exp-lab/girder,Kitware/girder,RafaelPalomar/girder,Kitware/girder,girder/girder,Kitware/girder,data-exp-lab/girder,RafaelPalomar/girder,girder/girder,girder/girder,kotfic/girder,data-exp-lab/girder,RafaelPalomar/girder,manthey/girder,girder/girder,manthey/girder,jbeezley/girder,kotfic/girder,RafaelPalomar/girder,kotfic/girder,Kitware/girder,jbeezley/girder,jbeezley/girder,manthey/girder,jbeezley/girder,kotfic/girder,RafaelPalomar/girder,manthey/girder,data-exp-lab/girder