Fix variable enclosure in /etc/hosts check

[andrewbolster]

In "Ensure hostname entries in /etc/hosts", the hostname_hosts_file is not properly enclosed for Ansible 2

[jschaul]

Hi, all the projects under yetu are unfortunately not maintained (and cannot be, since that company ceased to exist). All projects were forked; ansible-hostname can be found here: https://github.com/AutomationWithAnsible/ansible-hostname