Add support for disabling yum makecache and yum-cache-reload

We are currently creating yum repos and caching each repo, and then clearing all that cache at the end of the recipe.  This makes no sense and slows down the chef-client run significantly.  So I've added support to disable that.

Added 2 new attributes, default to nil:

['yum']['main']['makecache']
['yum']['main']['cachereload']

To get the old behavior back, set these 2 attributes to `true` (boolean).