Add staging

This adds playbooks for staging servers, which have certain variables and host groups that are different than development or contentqa.

The only change that will get interpreted in other environments is the addition of a PostgreSQL access control entry, which will not make any difference in development or contentqa.  There's a new playbook for the `ingestion` group, but that won't get run in the current contentqa or development environments.  It could be added to those later.