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.

Change-Id: If07d8e0802524f9babdee0c8cc07d40cb0803afb
Closes-bug: #1269722
Merge "Update help message of flag 'enable_isolated_metadata'"