metaplugin: use correct parameter to call neutron client

Use region_name instead of auth_region.

Change-Id: Ia4640f282de6d781f4f2d5b35da93afa4a88b6bb
Closes-Bug: 1252982
(cherry picked from commit 86617741bcbe040cc95f1fb733525895725279b3)
Merge branch 'stable/havana' of https://github.com/openstack/quantum into havana/stable