Pass SauceLabs credentials via capabilities

[spartanhooah]

SauceLabs recommends passing the credentials as part of the capabilities: https://wiki.saucelabs.com/display/DOCS/Best+Practice%3A+Use+Environment+Variables+for+Authentication+Credentials

I propose a change to the SauceLabsDriverFactory and/or the SauceLabs Gradle plugin that will append the provided credentials to the capabilities so they only have to be defined in one place in the Gradle buildscript.