Move variable-files to group_vars directory

The group_vars dir is the standard destination for ansible to look for
variable-files. So I do not have to declare them in the main.yml
anymore. This change will make it easier for me to reason about upcoming
changes, like using a vault for sensitive information.

ansible.cfg

@@ -4,3 +4,4 @@ inventory = inventory
 become = True
 ask-become-pass = True
 display_skipped_hosts = False
+error_on_undefined_vars = True

main.yml

@@ -1,9 +1,5 @@
 ---
 - hosts: localhost
-  vars_files:
-  - debian.config.yml
-  - macos.config.yml
-  - common.config.yml
   roles:
   - role: roles/macos
     tags: [macos]