Thanks to the commit c73b54e50b,
the way of DHCP agent determines how a subnet is isolated evolves.
But the flag help message wasn't updtated accordingly to this evolution.
Depends on https://review.openstack.org/#/c/77118/ getting merged.
Change-Id: If07d8e0802524f9babdee0c8cc07d40cb0803afb
Closes-bug: #1269722
(cherry-picked from 6dd1a370fe on master)