Generate secret projections for Knative serving

Changelog:

• Generate secret projections for Knative serving c9544f5

This commit projects secrets into the Knative CRD YAML, however
it appears that the CRD validator for the Service object will
now allow multiple volumes to be mounted into the same directory
which breaks compatiability with OpenFaaS on Kubernetes.

A separate directory is used for each secret with the key so
that "aws" will be mounted at /var/openfaas/secrets/aws/name.

$ cat stack.yml 
provider:
  name: openfaas
  gateway: http://127.0.0.1:8080  # can be a remote server
  network: "func_functions"       # this is optional and defaults to func_functions

functions:
  secret-print:
    image: functions/alpine:latest
    environment:
      fprocess: "cat /var/openfaas/secrets/alex/name"
    secrets:
     - alex
     - matias

Produces:

faas-cli generate --api serving.knative.dev/v1alpha1
---
apiVersion: serving.knative.dev/v1alpha1
kind: Service
metadata:
  name: secret-print
  namespace: openfaas-fn
spec:
  runLatest:
    configuration:
      revisionTemplate:
        spec:
          container:
            image: functions/alpine:latest
            env:
            - name: fprocess
              value: cat /var/openfaas/secrets/alex/name
            volumeMounts:
            - name: alex
              mountPath: /var/openfaas/secrets/alex
              readOnly: true
            - name: matias
              mountPath: /var/openfaas/secrets/matias
              readOnly: true
          volumes:
          - name: alex
            secret:
              secretName: alex
          - name: matias
            secret:
              secretName: matias