root_helper shouldn't be set in the agent ini files in grizzly

root_helper shouldn't be configured in the agent ini files. In
addition, root_helper should be configured under the AGENT section
in quantum.conf.

Change-Id: Ifa3320f6972450ff16a4cff7eab2397705d35f8b
Merge "root_helper shouldn't be set in the agent ini files in grizzly"