Allow to set/modify network mtu

This patch adds ``net-mtu-writable`` API extension that allows to write
to network ``mtu`` attribute.

The patch also adds support for the extension to ml2, as well as covers
the feature with unit and tempest tests. Agent side implementation of
the feature is moved into a separate patch to ease review.

DocImpact: neutron controller now supports ``net-mtu-writable`` API
           extension.
APIImpact: new ``net-mtu-writable`` API extension was added.

Related-Bug: #1671634
Change-Id: Ib232796562edd8fa69ec06b0cc5cb752c1467add
1 file changed
tree: 08e8c60d02aaf677dfa4f8e156bc10bc3bbc7f92
  1. neutron/