Added envValue & readFile to template functions.

[apollo13]

This allows for easier integration of docker secrets into config files. The rationale here is that environment variables can leak rather easily (docker inspect etc) whereas docker secrets are mounted directly into the container and not visible from the outside.

Added code complexity: small Backwards compatibility: 100%, only new functions got added, there is no behavioural change

[pkishino]

would love for this to me merged and released so we can use docker secrets..