Cisco Nexus: maximum recursion error in ConnectionContext.__del__

Cisco Nexus: maximum recursion error in ConnectionContext.__del__

If DevStack is configured for the Cisco Nexus plugin, the following
error is observed:

Exception RuntimeError: 'maximum recursion depth exceeded' in <bound
method ConnectionContext.__del__ of
<neutron.openstack.common.rpc.amqp.ConnectionContext object at
0x403a3d0>> ignored

The root cause of the problem is that the Cisco Nexus plugin's
PluginV2.__gettattr__ method, a model object is being passed
as a value for a unicode %s format mod. Because the neutron server
has "lazy gettext" (deferred interpretation of unicode objects) enabled,
this causes many layers of recursive calls to deepcopy.

The fix is to pass a string object for the unicode %s mod field.

Change-Id: I0a07a0ab417add68e44cb1bca722cb0b4a71205b
Closes-Bug: #1286565