in dhcp_agent, always use quantum.conf root_helper

Fixes bug #1178003.

For the most part, the dhcp agent uses the root_helper setting from the [AGENT]
section of /etc/quantum/quantum.conf. However, in a couple of spots, the
root_helper setting from the [DEFAULT] section of /etc/quantum/dhcp_agent.ini
was being used.

Change-Id: Id2f3140374a705cf7a0d2a784e4f321cc54f602e
Merge "in dhcp_agent, always use quantum.conf root_helper"