Fix namespace issue in generate_sample.sh
Fix namespace issue in generate_sample.sh

We just moved to using a hybrid approach that
uses the new oslo-config-generator for pulling out
library options to create the cinder.sample.conf file.
In that patch it was expected that we should use oslo_*
for the namespace.  That, however, is not how
oslo-config-generator works.

This patch switches to using oslo.* for the namespace to get
the library options pulled in.  This also fixes the fact that
we were putting multiple '[DEFAULT]' sections into the file.

Change-Id: Ie799d4d7e6deb2c486a93c5095fc3b766679fa95
Partial-bug:  1449235

apporc/cinder,scality/cinder,apporc/cinder,ge0rgi/cinder,scottdangelo/RemoveVolumeMangerLocks,tlakshman26/cinder-bug-fix-volume-conversion-full,bswartz/cinder,scality/cinder,takeshineshiro/cinder,JioCloud/cinder,winndows/cinder,bswartz/cinder,potsmaster/cinder,phenoxim/cinder,nikesh-mahalka/cinder,Nexenta/cinder,duhzecca/cinder,dims/cinder,tlakshman26/cinder-https-changes,saeki-masaki/cinder,openstack/cinder,nikesh-mahalka/cinder,cloudbase/cinder,leilihh/cinder,Paul-Ezell/cinder-1,manojhirway/ExistingImagesOnNFS,Hybrid-Cloud/cinder,CloudServer/cinder,phenoxim/cinder,tlakshman26/cinder-https-changes,potsmaster/cinder,eharney/cinder,tlakshman26/cinder-new-branch,petrutlucian94/cinder,petrutlucian94/cinder,mahak/cinder,tlakshman26/cinder-bug-fix-volume-conversion-full,duhzecca/cinder,mahak/cinder,eharney/cinder,leilihh/cinder,JioCloud/cinder,Datera/cinder,tlakshman26/cinder-new-branch,j-griffith/cinder,dims/cinder,j-griffith/cinder,NetApp/cinder,winndows/cinder,takeshineshiro/cinder,nexusriot/cinder,Nexenta/cinder,Datera/cinder,NetApp/cinder,sasukeh/cinder,saeki-masaki/cinder,cloudbase/cinder,scottdangelo/RemoveVolumeMangerLocks,Hybrid-Cloud/cinder,nexusriot/cinder,Paul-Ezell/cinder-1,CloudServer/cinder,sasukeh/cinder,manojhirway/ExistingImagesOnNFS,openstack/cinder