Make all secrets settable via files

[hairyhenderson]

With new Docker Swarm Secrets, and when running gomplate inside a container running in a Swarm service, it'd be more secure for gomplate to be able to read secrets from files instead of environment variables. See https://docs.docker.com/engine/swarm/secrets/#build-support-for-docker-secrets-into-your-images

I'm thinking of pretty much everything passed through an environment variable, but especially:

• VAULT_APP_ID
• VAULT_USER_ID
• VAULT_AUTH_GITHUB_TOKEN
• VAULT_AUTH_USERNAME
• VAULT_AUTH_PASSWORD
• VAULT_ROLE_ID
• VAULT_SECRET_ID
• VAULT_TOKEN - even though ~/.vault-token is already read by default, this one is still valuable since swarm secrets can only be exposed at /run/secrets (because they're in a tmpfs in-memory filesystem)

Basically each one of these would have an equivalent *_FILE version that would point to a file to be read.