Cisco plugin fails with ParseError no elem found

Cisco plugin fails with ParseError no elem found

When the Cisco nexus plugin is configured on DevStack, the tempest
Neutron API test test_port_list_filter_by_router_id fails with the
following error:
     ParseError: no element found: line 1, column 0

The root cause is that the Cisco Nexus plugin model layer
is encapsulating each dictionary response from the OVS subplugin
into a list, whereas the WSGI layer is expecting a dictionary
response. The fix is not to encapsulate these responses into a list.

Change-Id: I4b589e8e5696c0eae43cafc46912b3d7f29bbdb6
Closes-Bug: #1284162