Use renamed _fail_second_call() in cisco nexus tests

Use renamed _fail_second_call() in cisco nexus tests

In commit 5e4b0c6fc6670ea036d801ce53444272bc311929 NeutronDbPluginV2TestCase
_do_side_effect() was renamed to _fail_second_call(), but the nexus test cases
were not updated. The tests still worked because they check for
HTTPInternalServerError which is the end result of the server encountering
"AttributeError: object has no attribute '_do_side_effect'". So the fault
injection still worked, but not in the way intended.

Note: the monolithic nexus plugin is not updated here since it is being
removed from the tree.

Closes-bug: #1367552

Change-Id: I083285fff5144d8dc35bd5b64faa7e400c68bcff