Define default mode and device_id_string in Mount

self.mode is not a valid class member in the Mount abstract class,
but used in map_dev(). Although all the subclasses define it so
there is no runtime bug, define it to avoid confusion and
better readability.

Do the same thing for device_id_string as precaution too.

Partially fix bug 1037183

Change-Id: Ica32db1ca17c6ed967f2c310b1d54adeb5f4247c

nova/virt/disk/mount.py

@@ -30,6 +30,8 @@ class Mount(object):
     to be called in that order.
     """
 
+    mode = device_id_string = None  # to be overridden in subclasses
+
     def __init__(self, image, mount_dir, partition=None, device=None):
 
         # Input