switch to reader context

[zhanghao]

The mtu is already non-nullable, so the writer context can be switched to a reader context.

like this can be removed, and change it.
# NOTE(ihrachys) use writer manager to be able to update mtu
# TODO(ihrachys) remove in Queens when mtu is not nullable
with db_api.CONTEXT_WRITER.using(context):

In addition, redundant reader context can be removed.
def _get_subnet
    # TODO(ihrachys) remove explicit reader usage when subnet OVO switches
    # to engine facade by default
    with db_api.CONTEXT_READER.using(context):

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/701909
Committed: https://git.openstack.org/cgit/openstack/neutron/commit/?id=af274fa826fc97c15f9fe9b51a966caa8c5cf2eb
Submitter: Zuul
Branch: master

commit af274fa826fc97c15f9fe9b51a966caa8c5cf2eb
Author: zhanghao <email address hidden>
Date: Tue Jan 7 03:18:12 2020 -0500

    Switch to reader context

    Since the mtu is already non-nullable, the writer context
    can be switched to a reader context. In addition, the redundant
    reader context can be removed.

    Change-Id: I482a3892dee8ee94ab70142cf734f71e766178ed
    Closes-Bug: #1859258

[OpenStack Infra (hudson-openstack)]

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