Correctly load jitsi-defaults.properties when running under WebStart

When running under WebStart, using ClassLoader.getSystemResourceAsStream(fileName) does not work at all.

This is actually quite useful for somebody troubleshooting WebStart and this code only executes once at startup.  The code for detecting webstart is not 100% reliable, different people describe different techniques so it is really useful to know what type of environment Jitsi believes it is running in.