Adds root_helper parameter for DHCP Agent

Implements a workaround to address the following DHCP Agent bug:
   https://bugs.launchpad.net/quantum/+bug/1182616

Adds the root_helper parameter with the default setting of:
'sudo quantum-rootwrap /etc/quantum/rootwrap.conf'

Change-Id: If74c22c642c9f3d553901bf545fabc05f24520fe