Today, existing networks may not reflect MTU configured for
neutron-server, if they were created when neutron-server was using
different MTU setup for its infrastructure, or when it was using bad
default values for network MTUs (specifically, before Mitaka, all networks
were getting MTU = 0 by default, disabling both advertisement and data
path MTU size enforcement).
This patch stops persisting MTU in the database on network create and
instead calculate it on every network resource fetch.
DocImpact Now changes to MTU configuration options immediately affect
existing network MTUs, not just new networks.
UpgradeImpact Existing networks with invalid MTU persisted in database
may change their MTU values to reflect configuration.
Change-Id: Iee4f5037bf10b73ba98464143b183aacb59c22f2
Closes-Bug: #1556182