Fix a recent ipv6 UT regression

This fixes UT failures on systems without the procfs entry
by adding appropriate mocking.  The problem introduced by
the following change:
    https://review.openstack.org/#/c/116826/
    commit 0c963f6140237d8d3f361c6aaa7d2a6b656504db

Also, add a test for the case which the change in question fixed.

Closes-Bug: #1363805
Related-Bug: #1361542
Change-Id: I72842c3446f3275e2288adc411540ab036025ed0
(cherry picked from commit 6f5f14872387f3814d3c421f07e68f6ce45f7d7d)
Merge "Fix a recent ipv6 UT regression" into stable/icehouse