ansible-galaxy CLI: Allow 'init' to create a role even if the directory exists

This patch allows 'init' to create a role even if the desired role directory
already exists. If a directory matching the desired role_name exists, 'init' will then check whether the existing directory contains any ROLE_FILES. If the directory contains any files in ROLE_FILE, the --force argument will be required to initialize the role.  The patch corrects the initialization procedure documented at https://galaxy.ansibleworks.com/intro.