Add OPENFAAS_NS to provide a tenant's default namespace

Changelog for 0.18.13:

• PR #1028 Set the default namespace with OPENFAAS_NS by @welteki

Commits
4797310 Respect function namespaces when diffing stacks by @welteki
5860ddb Use OPENFAAS_NS as the default function namespace by @welteki
221106c Update alexellis/upload-assets to 0.5.0 by @alexellis
5a870ab Update examples for publishing multi-arch images by @alexellis

Changes: 0.18.12...0.18.13

Generated by Derek