allow activation of arbitrary plugins within a repository

Allows other plugins, other than just fastestmirror, to be used within a repository resource.

My particular use case is an s3 plugin - https://github.com/seporaitis/yum-s3-iam

Without this change the Chef developer is forced to use other means to generate the repo file - e.g. template resource - outside of the yum cookbook.

Usage example
yum_repository 'repo' do
  # ...
  plugins       ['s3' => true, 'foo' => false, 'bar' => true]
end