feat: Add multi-namespace test for scaling

[LucasRoesler]

Rewrite the scaling tests into a table driven test and add support to running in multiple namespaces.

Testing

kind create cluster

arkade install openfaas --basic-auth=false --clusterrole

kubectl create namespace certifier-test
kubectl annotate namespace/certifier-test openfaas="1"

export CERTIFIER_NAMESPACES=certifier-test

kubectl port-forward -n openfaas svc/gateway 8080:8080 &
make test-kubernetes

Resolves #70

[alexellis]

Sorry I missed this. Could you trigger a rebuild since the last 3 commits seem to have failed CI?