Add support for caching chef gems (in /opt/chef/embedded).

Dear @fgrehm , here is a modest PR to cache gems downloads within Chef's `/opt/chef/embedded` directory.

To give you a bit of context, I live in a country with limited Internet connectivity, which makes running `kitchen converge` quite painful. With this PR I can cache the installation of the `ci-reporter` and `chef-minitest-handler` gems, saving me 10 minutes each time I run either `kitchen test` or the initial `kitchen converge`.