extra_dhcp_opt extension should support update with value None

[ZhiQiang Fan]

we should support update extra_dhcp_opt with value None, if opt_value is None, then delete extra_dhcp_opt on that port

[ZhiQiang Fan (aji-zqfan)]

review: https://review.openstack.org/#/c/45927/

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/45927
Committed: http://github.com/openstack/neutron/commit/77aee9e71523c543286d05a2f553d4a30778d17c
Submitter: Jenkins
Branch: master

commit 77aee9e71523c543286d05a2f553d4a30778d17c
Author: dekehn <email address hidden>
Date: Tue Sep 10 10:13:29 2013 -0600

    Adds delete of a extra_dhcp_opt on a port

    Add support for delete of extra_dhcp_opt(s) on a port. Where
    [{'opt_name': 'opt_to_delete', 'opt_value': None}].

    Closes-Bug: #1228008

    Change-Id: I75baeff91575cac6546fe2cc6fcf7a0d8e92853f