Praefect subcommand for checking node connections

A subcommand should be added to the Praefect CLI tool that enables the checking of all backend nodes specified in Praefect's config file. This will make it easier to track simple mistakes.

For example, in our Praefect demo on 2019-12-13, there was a configuration issue due to using the public IP instead of the private VPC IP's. This meant that the listening port on the Gitaly node was restricted by the VM firewall settings. Having an easy to use diagnostic tool would have made finding this issue easier than manually reviewing each address.