Move remote_user to ansible.cfg

Command line options override config files like ansible.cfg, but
not inventory data or playbook directives. This allows us to over-
ride remote_user with `-u` on the command line to log in as root
before we have set up the provisioning user.

See: https://github.com/ansible/ansible/issues/7841#issuecomment-245716271

web.yml

@@ -3,7 +3,6 @@
 
 - name: Configure web servers
   hosts: web
-  remote_user: provisioning
   become: yes
   roles:
     - common