Add a AppRTCDemo setting to change the GAE server.

BUG=4041
R=tkchin@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/36599004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7966 4adac7df-926f-26a2-2b94-8c16560cd09d