We should provide a set of task that makes it easy to deploy knative services using Tekton, related to tektoncd/pipeline#250, tektoncd/pipeline#303

An initial attempt is done in #34, but we need more:

• Using knative/client instead
• Provide more parameters and features (more than just deploy, we may want to change some environment variables, do some blue/green deployement and thus change the routing ratio, …)

cc @imjasonh