Update puppetDB docs to include info on LibPQFactory

Prior to this commit, the docs only enabled ssl for postgresql
connections. This commit changes the docs to add a few extra lines in
the database.ini file to use host cert/key and CA cert for verification
with ssl.
Yeah, it looks like we're going to be able to use the certs and keys that puppet gives us for this no problem. I think the only time the CN mattered is if you set sslmode=verify-full. Then it needed to be a root CA and also have the hostname match the cert. At the moment I just did verify-ca, which only verifies the CA chain (if there even is one).