Fix attribute deletion when updating config

The received network parameter name can have a ':json' suffix that
is not present in the class attribute of this parameter.
This suffix was not removed and could cause attribute deletion to fail.

/reviewed-on nexedi/re6stnet!20

re6st/cache.py

@@ -121,7 +121,7 @@ class Cache(object):
                     old[k] = v
                     continue
                 try:
-                    delattr(self, k)
+                    delattr(self, k[:-5] if k.endswith(':json') else k)
                 except AttributeError:
                     pass
                 remove.append(k)