Run Sample Apps as Non Root

When testing out the guestbook sample app on a cluster in a restricted environment, the deployment fails due to the guestbook [container being run as root](https://github.com/argoproj/argocd-example-apps/blob/6bed858de32a0e876ec49dad1a2e3c5840d3fb07/guestbook/guestbook-ui-deployment.yaml#L16). It would be nice to specify a securityContext for the deployment since it should not need privileged permissions.

While this is a sample app, it would be nice to promote the practice of not running containers as root and also help to prevent running into the issue of the Getting Started tutorial not working in certain Kubernetes environments.

Start with guestbook/guestbook-ui-deployment.yaml at the linked container definition and review how the deployment runs the guestbook UI. Update the deployment's security settings so it works in restricted Kubernetes environments, then verify that the guestbook sample and Getting Started tutorial deploy successfully without running the container as root.

Written by the indexing model from the issue text.