Clean up IPV6 config checks in test_wsgi.
Clean up IPV6 config checks in test_wsgi.

Two tests in test_wsgi require a configured IPV6 interface.
We added a check/skip for this but it wasn't very graceful and
didn't work on OS-X or platforms other than Linux.

This patch implements the checks a bit cleaner using the skip_if
decorator, and combines the linux/path check and configured interface
check.

Change-Id: I950fdfeb8135912faf5dcaabe52fb87abd73dd1e

ntt-sic/cinder,redhat-openstack/cinder,nikesh-mahalka/cinder,spring-week-topos/cinder-week,duhzecca/cinder,cloudbase/cinder,openstack/cinder,winndows/cinder,Thingee/cinder,abusse/cinder,eharney/cinder,eharney/cinder,tlakshman26/cinder-new-branch,rickerc/cinder_audit,leilihh/cinder,tlakshman26/cinder-bug-fix-volume-conversion-full,cloudbau/cinder,Thingee/cinder,Datera/cinder,tlakshman26/cinder-https-changes,NeCTAR-RC/cinder,manojhirway/ExistingImagesOnNFS,phenoxim/cinder,Datera/cinder,Hybrid-Cloud/cinder,sasukeh/cinder,inkerra/cinder,redhat-openstack/cinder,saeki-masaki/cinder,blueboxgroup/cinder,leilihh/cinder,scality/cinder,tlakshman26/cinder-bug-fix-volume-conversion-full,JioCloud/cinder,NeCTAR-RC/cinder,ge0rgi/cinder,apporc/cinder,citrix-openstack-build/cinder,CloudServer/cinder,Paul-Ezell/cinder-1,hguemar/cinder,rakeshmi/cinder,duhzecca/cinder,blueboxgroup/cinder,apporc/cinder,github-borat/cinder,mahak/cinder,cloudbase/cinder,openstack/cinder,Hybrid-Cloud/cinder,NetApp/cinder,manojhirway/ExistingImagesOnNFS,j-griffith/cinder,julianwang/cinder,github-borat/cinder,potsmaster/cinder,nexusriot/cinder,saeki-masaki/cinder,hguemar/cinder,takeshineshiro/cinder,Nexenta/cinder,scottdangelo/RemoveVolumeMangerLocks,spring-week-topos/cinder-week,citrix-openstack-build/cinder,phenoxim/cinder,CloudServer/cinder,maelnor/cinder,dims/cinder,nexusriot/cinder,Akrog/cinder,rickerc/cinder_audit,JioCloud/cinder,bswartz/cinder,alex8866/cinder,julianwang/cinder,NetApp/cinder,sasukeh/cinder,ntt-sic/cinder,tlakshman26/cinder-new-branch,Thingee/cinder,scottdangelo/RemoveVolumeMangerLocks,inkerra/cinder,winndows/cinder,petrutlucian94/cinder,scality/cinder,dims/cinder,takeshineshiro/cinder,mahak/cinder,Accelerite/cinder,Accelerite/cinder,alex8866/cinder,tlakshman26/cinder-https-changes,j-griffith/cinder,Nexenta/cinder,cloudbau/cinder,rakeshmi/cinder,potsmaster/cinder,Paul-Ezell/cinder-1,nikesh-mahalka/cinder,Akrog/cinder,bswartz/cinder,abusse/cinder,citrix-openstack-build/cinder,petrutlucian94/cinder,maelnor/cinder