As described inline, we should lower the MTU in the docker
configuration when we see the interface has a MTU lower than 1500 so
things "just work". This particularly affects the Linaro ARM64 cloud
in OpenDev, but it is a generic issue.
Change-Id: I338616c41a65b007d56648fdab6da2a6a6b909f4
Story: https://storyboard.openstack.org/#!/story/2008230