Fully declarative deployment configs for Google Cloud

Our deployment process currently involves running shell scripts in examples/deployment/kubernetes, which use a combination of tools to build Docker images, setup a Kubernetes cluster on GCP, configure it and deploy Trillian. This could be improved by using a tool such as [Terraform](https://www.terraform.io/) to replace the shell scripts and either relying on [our published Docker images](https://gcr.io/trillian-opensource-ci) or using Google Cloud Build to create the images. [Kustomize](https://kustomize.io/) could be used to support a wider variety of deployments (currently, we use envsubst for a similar purpose, but it is less flexible).