Feature/add external vars configuration

NOTE: I recommend reviewing and eventually merging pull request [https://github.com/glenjarvis/ansible_aws_playbooks/pull/10] first before reviewing this request.

Configuration should be successful for `external_vars.yml`. However, one would need a genuine copy of external_vars.yml to be able to play this playbook. There are several private variables that are needed for these playbooks. For example:

[For accessing s3 bucket]
* ansible_deploy_aws_access_key
* ansible_deploy_aws_secret_key

[For configuring deploy]
* git_deploy_public_key
* git_deploy_private_key

These are not specific to individuals, but to the playbooks we are deploying (e.g., retrieving a file from s3 bucket during the deploy).