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

openrc/pgmon.initd

@@ -19,6 +19,6 @@ start_pre() {
 }
 
 command="/usr/bin/pgmon"
-command_args="'$CONFIG_FILE'"
+command_args="-c '$CONFIG_FILE'"
 command_background="true"
 command_user="${PGMON_USER}:${PGMON_GROUP}"