Add metadata.rb for use with librarian-chef

I essentially did this in the `worker_host` and `ci_environment` directories:

```bash
for cookbook in *; do [ -e $cookbook/metadata.rb ] || cat <<-METADATA > $cookbook/metadata.rb; done
maintainer       "Travis CI Development Team"
license          "Apache v2.0"
description      "Installs/Configures $cookbook"
METADATA
```