Feature/update ansible hosts to three tiers

NOTE: It is recommended to review pull requests https://github.com/glenjarvis/ansible_aws_playbooks/pull/10 and https://github.com/glenjarvis/ansible_aws_playbooks/pull/11 before this pull request (as it builds upon the same work).

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.