add webdriver property to pom.xml, e.g. chrome or firefox

pom.xml now has a "webdriver" property (default is chrome, other value for now is firefox, easy to add more in the future). The UI tests pick up that property and load the right driver. I tried it with both chrome and firefox on Windows (didn't try ubuntu, but should work). I also took the opportunity to clean up the properties-handling code a bit.