[fix] Fixed translation initialization issue

Seems expressions like f' {_("ms")}' force the evaluation
of the translatable string, but the translation system
is not ready at this stage of the app startup yet and
django throws an error in production.

openwisp_monitoring/monitoring/configuration.py

@@ -86,7 +86,7 @@
                     _('Average Max RTT'),
                     _('Average Min RTT'),
                 ],
-                'unit': f' {_("ms")}',
+                'unit': _(" ms"),
                 'order': 220,
                 'query': chart_query['rtt'],
             },
@@ -173,7 +173,7 @@
                     _('Total download traffic'),
                     _('Total upload traffic'),
                 ],
-                'unit': f' {_("GB")}',
+                'unit': _(" GB"),
                 'order': 240,
                 'query': chart_query['traffic'],
             },