[COOK-3968] Support jenkins user's $HOME being different than $JENKINS_HOME

https://tickets.opscode.com/browse/COOK-3968

Though it is a good pattern to have the $JENKINS_HOME be the same as the dedicated jenkins user's $HOME, there are some cases that it doesn't make sense. (think corporate IT)

I did my best to make this change backwards compatible. I also added documentation to the README and added some test kitchen suites. It should also pass foodcritic.