volume puppetdb-ssl doesn't exist.

## Describe the Bug
Volume puppetdb-ssl is referenced in the documentation but not created when stack is started.

## Expected Behavior
A volume should exist for the puppetdb-ssl for container puppetdb instance based on the README documentation.

## Steps to Reproduce
docker-compose up -d
docker volume ls  <- Notice no volume for puppetdb-ssl

## Environment
Docker 20.10.3
Ubuntu 20.04
