Default value of connector_plugindir can cause problems

The default value for connector_plugindir is /usr/lib64/mysql/plugin which normally contains plugins for the server. If the server requests authentication using a plugin and that plugin exists in the default location, it is possible that an unsupported version of the plugin is used.

If we want to support authentication of monitor connections with different client-side plugins, the plugins built for the version used by MaxScale should be included in the package itself. If we don't want to support it (or before we include them in the package), the path should point somewhere where we know the plugins won't be.