ConsulBackendPlugin - allow environment variable for address

[kmanning]

• All of Vipar's terraform currently use Consul as the backend
• The Consul address is hardcoded across all the projects
• What if instead, we simply had the ConsulBackendPlugin search for a DEFAULT_CONSUL_ADDRESS environment variable, and inject that as a parameter if found. That would reduce the amount of duplication across all the projects, and make it easier to migrate to a different consul if every necessary.