Checking users YAML settings

It is easy to break things with bad choices in the host_vars/host.example.org.yml file, can some Molcule test or some other tests be added to be run using CI, for example:

• Unique system usernames, users
• No duplicate database names, users_mariadb_databases
• No duplicate users_apache_server_name or users_apache_server_aliases
• Make some variable compulsory