Add namespace field to the stack.yml schema

[alexellis]

Add namespace field to the stack.yml schema

Expected Behaviour

I'd like to write this:

functions:
  webhook:
    namespace: staging-fn

Then run faas-cli deploy, and have the function deployed into the staging-fn namespace.

Current Behaviour

Functions can now be deployed into a custom namespace via faas-cli deploy --namespace, but not with a YAML file.

Possible Solution

Extend the structs / schema.

Some of the deployment paths are already up to date from when we added --namespace in #693 so this shouldn't need wide-spread changes.

This work is part of: openfaas/faas-netes#511

[alexellis]

This appears to be done already, a user reported that it wasn't available, but I've tested e2e with the latest faas-cli and it worked fine.