Reset the IPv6 API backend when resetting the conf stack.
Reset the IPv6 API backend when resetting the conf stack.

This fixes test isolation errors where tests were running with
the wrong ipv6 backend and failing. The failures occured because
Python discovers tests in filesystem order rather than a sorted
order, and the test that was changing the backend had previously
always run later under nose, which does not use filesystem order.

Change-Id: Ife4370d0747b269c9bc9aabb50c531cb6c9fec13

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