By default, the cpanm client will use a default mirror to download
libraries, but it's possible for the end user to pass a different URL.
Since ansible favorite idempotence when possible, we verify if the
module is already installed before running the cpanm client.
Another minor additional change, the `notest` option is now a boolean.
cpanm is a popular alternative to the CPAN client for installing Perl
libraries. Unfortunately it can't uninstall already installed module, so it
can't handle states.