set a fixed version of installed WP-CLI

The recommended method for installing WP-CLI is to download the last current version from URL https://raw.githubusercontent.com/wp-cli/builds/gh-pages/phar/wp-cli.phar .

So it is impossible to have a coherent puppet module in a long time.

One method is proposed with composer to install a specific version composer create-project wp-cli/wp-cli:0.22.0 --no-dev. This method was got from https://make.wordpress.org/cli/handbook/installing/