Add support to specify secrets in services

**What**
- During function creation, accept an array of strings defining swarm secrets
that are required for the service
- Update docs
- Add new guide on using the secrets capability
- Add new sample function to highlight using environment variables
- Update `ApiKeyProtected` sample function to utilize the new secrets
capabilities

**Why**
- This allows secrets to remain encrypted at rest instead of being unencrypted
in environment variables and yaml files.

Fixes #285

Signed-off-by: Lucas Roesler <lucas.roesler@gmail.com>

sample-functions/NodeHelloEnv/build.sh

@@ -0,0 +1,3 @@
+#!/bin/bash
+
+docker build --squash -t functions/nodehelloenv .