Delete secrets and sensitive information after deployment

[yeoldegrove]

This builds the feature suggested in #862.

To delete e.g. /etc/salt/grains and other sensitive information from the hosts after a successful deployment, you can set cleanup_secrets = true in terraform.tfvars. This is disabled by default to make debugging easier.

If enabled, currently the following files are deleted after a successful deployment:

• /etc/salt/grains
• /var/log/salt-os-setup.log
• /var/log/salt-predeployment.log
• /var/log/salt-deployment.log
• /var/log/salt-postdeployment.log