Allow for demo deployments to optionally include the reads deployment

[sjahl]

It's now a common occurrence for us to be deploying browser demos on clusters that are not the production cluster. In many of these cases, there is no reads deployment, unless reads is the component that is being demoed. This creates friction when trying to create the demo ingress, since it expects there to be a reads service. The workaround is simply to attempt to create the ingress again, which works a second time, because the first attempt creates the (bogus) reads Service objects before it ultimately fails.

We should have an option in our demo ingress / kustomize config to only create the reads service when desired.