Long story short, we auto-clamp down everything to 1400 bytes
due to VXLAN tunneling for multinode testing. But there are other
reasons to clamp it smaller, and we will need to clamp that further
for multinode should we mix it with OVN.
Anyway, this should make things cleaner and we should rely upon the
gate calcualted MTU as a starting place, not the guess based upon
interface list. i.e. test VM could be wrong but gate could know better.
Change-Id: I385679fe30d1447f1ed94cdf5a419e6acefbc595
40e825ba93