Add query test script and test mode

* Add a mode to test all metric queries

* Add a script to run query tests against different versions of
  PostgeSQL

* Add Docker elements for query testing

* Switch to using a --config flag when specifying the config file

* Fix some metric queries

* Allow the agent address to be configured

* Allow the sslmode connection parameter to be configured

systemd/pgmon@.service

@@ -7,7 +7,7 @@ After=network.target
 [Service]
 EnvironmentFile=/etc/pgmon/%i-service.conf
 User=${SERVICE_USER:-postgres}
-ExecStart=/usr/local/bin/pgmon /etc/pgmon/%i.yml
+ExecStart=/usr/local/bin/pgmon -c /etc/pgmon/%i.yml
 ExecReload=kill -HUP $MAINPID
 Restart=on-failure
 Type=exec