Remove remaining oslo-incubator code from Cinder

Remove remaining oslo-incubator code from Cinder

Most of the parts for oslo-incubator have moved to
oslo libraries.  Cinder only had imageutils.py and
_i18n.py remaining in cinder/openstack/common.  I
was to able remove both of these remaining incubator
components.

oslo_utils now has imageutils available.  I moved
cinder/image/image_utils.py to use the new
imageutils from oslo_utils and updated all the other
places that were using the old imageutils import.

_i18n was only being used in scheduler/base_filter.py
which appears to have been accidentally left when the
scheduler was previously pulled out of incubator.  I
fixed this oversight.

Finally, this patch deletes cinder/openstack/common
cinder/openstack and the openstack-common.conf file.

Change-Id: I8fe0b5d6c278715620f93f682289df7ae60dbeba
Closes-bug: 1517861