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
Raise max header size to accommodate large tokens

The max header is exceeded in the following scenario
- Auth tokens built with a keystone v3 API catalog
- A catalog with approximately 8 or more endpoints defined

Change-Id: Ie815e457c0b25ab828e780b5d90233ba0ceff61f
Closes-Bug: #1251026
Merge "Update help message of flag 'enable_isolated_metadata'"
Merge "Raise max header size to accommodate large tokens"