Add config option to identify buckets that should not be automatically
enabled with using `config.cache.auto_detect = true`.
After turning on the auto_detect behaviour, with this change you can
now explicitly disable any bucket type through your Vagrantfile e.g.
`config.cache.disable = :chef_gem`.
Fixes#115
This bucket must be explicitly enabled and may be useful for users
wanting a generic caching mechanism that they can use directly instead
of via some predefined packaging system.
For instance, this enables one to implement a cache for packages
downloaded via wget or curl.
Doing this is kinda irresponsible because we don't have any unit testing
in place but I've had enough of copy & pasting things around. Although it
doesn't make the codebase GREAT, I believe it'll reach a _nice_ status :)
Thanks to those changes I realized that skipping configuration of
buckets that have already been configured was easier than I thought and
should be enough to close GH-85 \o/
