Correct PXEPrivateMethodsTestCase.setUp

Unit tests within the PXEPrivateMethodsTestCase test class have been
sporadically failing on py26 test nodes since f14dacb7. That commit
introduced tests that interacted with the DB without inheriting from
DbTestCase, and loaded a driver via stevedore without mocking the
extension manager loading.

This patch corrects the above ommissions.

Change-Id: I2f930db7d3bb8abe120ce24a604f800c2e861fca
Closes-bug: #1279992