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
ML2: Fix release of network segments to allocation pools

The recent merge of https://review.openstack.org/#/c/115025/ broke the
release of network segments to allocation pools on network delete,
resulting in network creates eventually failing with
NoNetworkAvailable exceptions. The network_delete() code is rearranged
to properly release the network segments, and a unit test is updated
to verify that the type drivers' release_segment() method is called.

Partially implements: Blueprint ml2-type-driver-refactor

Change-Id: Ica5f11b2012dedfcc097f34f1515f5f3ca208fd2
Merge "ML2: Fix release of network segments to allocation pools"
Merge "Fix a recent ipv6 UT regression"