turn classmethod 'find_all_plugins' into an instance method.

change 'use_' prefix to 'enable_plugin_'

tests/regtest.py

@@ -82,7 +82,7 @@ class TestLightningSwapserver(TestLightning):
         },
         'bob': {
             'lightning_listen': 'localhost:9735',
-            'use_swapserver': 'true',
+            'enable_plugin_swapserver': 'true',
         }
     }