Clean up openstack-common.conf

oslo-incubator will introduce new integrity check in [1].
That will help developer find inconsistent issue while copying
from oslo-incubator. The check just outputs warnings. These are
changes according to warning:
 * remove unused module py3kcompat
 * add missing module context, processutils in openstack-common.conf
 * sort the modules in openstack-common.conf
[1] https://review.openstack.org/#/c/76901/

Change-Id: I2fea01148f6ad268a5fcfdaa788e30d6fcb48c3f
Related-Bug: #1295582
Merge "Clean up openstack-common.conf"