[Feature Request] support deployment specification params formatting in appspec.yml

[mwjones-aws]

#70

Copied from PR description

It looks like it is possible to use deployment specification values in hook scripts
https://blogs.aws.amazon.com/application-management/post/Tx1PX2XMPLYPULD/Using-CodeDeploy-Environment-Variables

However, I noticed that it wasn't possible to use these values in the appspec.yml file. This PR adds support for that. To use the deployment specification values in the appspec.yml you can simply do this:

version: 0.0
os: linux
files:

• source: someDir
  destination: /var/www/<%= application_name %>
  hooks:
  ValidateService:
  • location: scripts/validate_app
    timeout: 15
    runas: root

The following values are supported:

application_name
deployment_id
deployment_group_name
deployment_group_dir
deployment_root_dir
last_successful_deployment_dir