Add secrets support to provider

Changelog for 0.7.0:

• PR #32 Add secrets support (with flat files) to the provider by @alexellis

Adds secrets support and binding of secrets at runtime to
functions. Files are written in plain-text to a 0644 permission
folder which can only be read by root and the containers
requesting the secret through the OpenFaaS API.

Tested by deploying an alpine function using "cat" as its
fprocess.

Happy to revisit at a later date and look into encryption at
rest. This should be on-par with using Kubernetes in its
default unencrypted state.

Fixes: #29

Generated by Derek