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)
NSX: Fix possible deadlock in sync code

This patch removes the call to nsx from within the db transcation
which can cause deadlock if another request comes in during this time.

Closes-bug: #1278991
Change-Id: If16b53a77997bccfdff2554dead26f42e23eaec9
(cherry picked from commit d59d5a1c72c2324b857225680f5578d381a2ec73)
Merge "Update help message of flag 'enable_isolated_metadata'" into stable/havana
Merge "NSX: Fix possible deadlock in sync code" into stable/havana
Merge branch 'stable/havana' of https://github.com/openstack/quantum into havana/stable