[doc] Add MTU information in DB and API

[OpenStack Infra]

https://review.openstack.org/349506
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commit e3afcd751ff8e681867e1c7a054684277ec2874b
Author: Marc Koderer <email address hidden>
Date: Thu Aug 4 15:01:29 2016 +0200

    Add MTU information in DB and API

    MTU value can be different for each neutron network.
    E.g. for high-performance use-cases it's very important to also
    support MTU's with jumbo frames.

    This patch exposes this information to the drivers. Each driver
    should setup its resources accordingly.

    The tempest test actually just covers the api change. Better coverage will
    be added when the container driver lands.

    APIImpact
    DocImpact

    Change-Id: I9b4efae620ec9f6790547c8fffc58872d43277f5
    Implements: bp add-network-mtu
    Related-Bug: #1612528

[Tom Barron (tpb)]

https://review.openstack.org/#/c/514498/ updated the api-ref. Admin do still needs update.

[Jason Grosso (jgrosso)]

https://review.openstack.org/#/c/514498/ was merged api-ref was updated