Add assertion to test_page_reverse method

In neutron/tests/unit/test_api_v2.APIv2TestCase.test_page_reverse
there is no second call to test_case with 'page_reverse': 'False,
because of use assert_called_once_with.
In proposed change before second testcase, reset_mock is called.
Remove also second 'instance' initialization.

Closes-Bug: #1373126
Change-Id: I552b7b47456a1cdfccdeb8d7853ec3395317097a
Merge "Add assertion to test_page_reverse method"