Make the MTU attribute not nullable

[zhanghao]

mtu attribute is nullable in the network, address all such TODOs
# TODO(ihrachys) remove in Queens+ when mtu is not nullable

then,we can also use reader context instead of writer context in some methods.

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/679399
Committed: https://git.openstack.org/cgit/openstack/neutron/commit/?id=68625686a40b3eb75502c8116f23d2297e288ca1
Submitter: Zuul
Branch: master

commit 68625686a40b3eb75502c8116f23d2297e288ca1
Author: zhanghao <email address hidden>
Date: Sat Oct 26 16:57:27 2019 -0400

    Make the MTU attribute not nullable

    This patch sets the MTU attribute to non-nullable, the code
    that get the MTU and update the network in some methods can
    be removed. If the MTU is empty before the pike version, it
    is set to the default value of 1500.

    Change-Id: Id4d738dde7fa4b7caccabad0aac542b82b4d7af1
    Closes-Bug: #1842261

[OpenStack Infra (hudson-openstack)]

Related fix proposed to branch: master
Review: https://review.opendev.org/701380

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/701380
Committed: https://git.openstack.org/cgit/openstack/neutron/commit/?id=08bfbed4c35e0aab95d44619dd7975d969d8acba
Submitter: Zuul
Branch: master

commit 08bfbed4c35e0aab95d44619dd7975d969d8acba
Author: zhanghao <email address hidden>
Date: Mon Jan 6 11:54:12 2020 -0500

    Remove the condition that the mtu is nullable

    The mtu is already non-nullable, it no longer needs to
    judge the mtu when updating the network.
    Related-Bug: #1842261

    Change-Id: Ic097bc2ae50198a3bda45e4a8161c22190ef1964

[OpenStack Infra (hudson-openstack)]

This issue was fixed in the openstack/neutron 16.0.0.0b1 development milestone.