remove kernel override ability

This was landed to try to address an issue with netns vs nbd during
icehouse development. It never really got us anywhere, and is now
just cruft.

Change-Id: Ie498fae8d1f796c1fc83459c65d0de948d1d50ce
Merge "remove kernel override ability"
Merge branch 'master' of https://github.com/openstack-dev/devstack