Enhance geb support for cloud browser testing

[aybartsch]

According to the Book of Geb chapter 14 I'v applied both the geb-saucelabs and the geb-browserstack plugins in build.gradle and driver creation via factories in GebConfig.groovy. Beside of others I'm facing two main issues:

geb-browserstack

• It seems that one cannot define connect presets like tunnel-identifier in gradle. There is just the option forceLocal. Please, provide connect presets for browserstack as it is available for saucelabs.

geb-saucelabs

• The SauceLabsDriverFactory is currently just supporting the URL for the US hub but I'm registered for the new EU hub https://ondemand.eu-central-1.saucelabs.com/wd/hub. Please, add support to define the URL used for driver creation.