ParseToken should also allow for environment variables

**Describe the feature you'd like**
I would like the Docker environment variable to work interchangeably in credentials-file

**Describe alternatives you've considered**
Right now it requires an annoying number of steps to base64 decode, and manually adjust json to allow docker to use a credentials-file vice an environment variable

**Additional context**
Right now the tutorial for Dockerfile uses the dictionary a,t,s to represent AccountTag, TunnelID, TunnelSecret; and a different structure (TunnelToken) is used to generate the Credentials through TunnelToken.

A few options: readTunnelCredentials in cloudflared/cmd/cloudflared/tunnel/subcommand_context.go should accept either dictionary as the appropriate variables (or support another base 64 decode). Or, the environment variable should be a valid variable within config.yml.