erroneous config files should be accepted in command line

frappyman.is_cfg(config) should not try to run the cfg file

frappyman.py

@@ -227,11 +227,11 @@ class FrappyManager(ServiceManager):
         raise FileNotFoundError(f'{cfg} not found')
 
     def is_cfg(self, ins, service, cfg):
-        try:
-            self.cfg_details(ins, service, cfg)
-            return True
-        except Exception:
-            return False
+        for cfgdir in self.config_dirs(ins, service):
+            cfgfile = join(cfgdir, f'{cfg}_cfg.py')
+            if exists(cfgfile):
+                return True
+        return False
 
     def all_cfg(self, ins, service, details=False):
         """get available cfg files