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
I suggest leaving major_platform_version alone, versus modifying it, and using another variable. Also, this doesn't allow for the same usage on RHEL as Debian, due to this defaulting. Maybe, instead, we should do something like:

```ruby
case major_platform_version
when 5, 6
  ver = major_platform_version
when 2014
  ver = 6 # for amazon linux
else
  log 'version warning' do
    message 'Unknown operating system version specified'
    level :warn
  end
  ver = major_platform_version
end
```