Update ansible_hosts to typical three tier system

Most projects have three tiers (development, staging and production). The
ansible_hosts file has been updated to reflect this system (e.g.,
production_webservers, staging_webservers, and development_webserver).

To also allow better management, DNS names have been included. Instructions for
overriding the `/etc/hosts` file during development has been included in the
README.
Merge pull request #14 from glenjarvis/feature/update_ansible_hosts_to_three_tiers_r1

Update ansible_hosts to typical three tier system