Fix 404 on ARender preview

Preview of Picture and Video documents return a 404 error.

Steps to reproduce:

• Set up date-based arender rendtion docker image:

  docker run -p 8761:8761 --name arender_rendition -it dockerin-arender.nuxeo.com:443/arender-rendition:1.0.1-I20181204_1154
  

• nuxeo.conf:

  org.nuxeo.connect.url=https://nos-preprod-connect.nuxeocloud.com/nuxeo/site/
  arender.server.rendition.hosts=http://localhost:8761
  arender.server.previewer.host=http://localhost:8080
  nuxeo.jwt.secret=test
  

• Install nuxeo-web-ui and nuxeo-dam.
• Create a Picture document.
• Navigate to the Picture document and click on the Annotations tab.
  => 404 error displays on screen.
  => Error in browser console:

  nuxeo-app.html:4106 GET http://localhost:8080/ARender/?uuid=Not%20Found 404