Feature/bigtop support

fixes COOK-1 https://issues.cask.co/browse/COOK-1
- [x] support for apache bigtop distribution
- [x] ability to specify custom repos without gpg key validation
we are already inconsistent between hdp (default to 6), cdh (do nothing), and this would be a third variation that introduces a y2015k bug.  I'll make it consistent with hdp, using another variable.