Ensure that debug agent has root_helper in [AGENT] section

This is a temporary fix until the Quantum patch lands. Then
devstack will be updated to remove all of the root_helper
settings in the DEFAULT section.
The Quantum patch in Question is:
https://review.openstack.org/#/c/20603/

Change-Id: Ie17ae76e8b525481e1002bd941812390cf2e2afc
Merge "Ensure that debug agent has root_helper in [AGENT] section"