Missing configuration opts from cinder.sample.conf

Missing configuration opts from cinder.sample.conf

The logic for writing opts registered with register_opt() to the
auto-generated opts.py file was flawed in that the directories where
there were opts being registered with this method would be imported,
but the actual opt would be missed when looking through the file for
the name of the opt being registered. Now the singular opts are being
caught and written to opts.py for the oslo-config-generator to process.

The 'backend' section was added to generate_cinder_opts.py
due to some of the missing opts being a part of that section.

Also the way some of the opts that were being registered with groups
were incorrect and so this addresses those in both how they were
processed in generate_cinder_opts.py and the files in which they are being
registered.

There is also one change to the name of an opt in cinder/volume/api.py.
Instances such as this one will be caught by the in-progress hacking
check patch: https://review.openstack.org/#/c/223375/

Change-Id: I8d333d7529d40f3a3b3651ca3a52c0048d96b99a
Closes-Bug: #1498122