fix: allow config directory to be configurable

For postgres, the config directory doesn't actually exist, so adding this configurability allows the plugin's info command to report correctly.

config

@@ -21,6 +21,7 @@ export PLUGIN_SCHEME="postgres"
 export PLUGIN_SERVICE="Postgres"
 export PLUGIN_VARIABLE="POSTGRES"
 export PLUGIN_BASE_PATH="$PLUGIN_PATH"
+export PLUGIN_CONFIG_SUFFIX="data"
 if [[ -n $DOKKU_API_VERSION ]]; then
   export PLUGIN_BASE_PATH="$PLUGIN_ENABLED_PATH"
 fi