Clean up openstack-common.conf

The issues are detected by new change about update.py
in I68c0108304be8a315bcefe939c22019caab9bc98. This patch
doesn't depend the oslo-incubator patch.
 * remove unused module eventlet_backdoor, service,
   threadgroup, loopingcall.
 * add missing module excutils, processutils inopenstack-common.conf

Change-Id: Ice2acb3ff73111858f5cf39e8fbdfb7db4b7c538
Merge "Clean up openstack-common.conf"