Replace singleton lists with None defaults

Hit an issue trying to run zuul-bwrap locally and I didn't pass
--ro-bind or --rw-bind which meant setMountsMap was being passed None
values. In fixing that, it seems to me that perhaps the list values were
not intentionally singletons.

However - it's possible they were and I'm breaking some intended logic
here.

Change-Id: I3a30bd3d4439c27483c45f86d3d9ae1741a40a38

zuul/driver/bubblewrap/__init__.py

@@ -83,7 +83,11 @@ class BubblewrapDriver(Driver, WrapperInterface):
     def stop(self):
         pass
 
-    def setMountsMap(self, ro_dirs=[], rw_dirs=[]):
+    def setMountsMap(self, ro_dirs=None, rw_dirs=None):
+        if not ro_dirs:
+            ro_dirs = []
+        if not rw_dirs:
+            rw_dirs = []
         self.mounts_map = {'ro': ro_dirs, 'rw': rw_dirs}
 
     def getPopen(self, **kwargs):