The default MTU for the management interface is sometimes bigger than
allowable by the physical infrastructure. Make the MTU configurable in
cloud-init and via config json. For cloud-init default it to the minimum size
for IPv6 if the value is not specified in boot command.
Change-Id: Ib4d4381f6977aabbeefd2f520bb5fc26ea54ffcd
Closes-Bug: #1539786