Update help message of flag 'enable_isolated_metadata'

Thanks to the commit c73b54e50b62c489f04432bdbc5bee678b18226e,
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 6dd1a370fe2ece809c06202c04a7d230a74fa91a on master)
Merge "Update help message of flag 'enable_isolated_metadata'" into stable/havana