`install_options` has special semantics (https://github.com/puppetlabs/puppet/blob/3.4.3/lib/puppet/type/package.rb#L307-L328) where install_options can take an array of Strings or Hashes, where the hashes are key/value pairs like `{'--prefix' => '/tmp'}`; this code will need to respect this sort of thing. Since the rpm provider already respects this functionality (https://github.com/puppetlabs/puppet/blob/3.4.3/lib/puppet/provider/package/rpm.rb#L155-L177) and the rpm provider is the parent class of the yum provider, I believe that we can directly use `#join_options` here.