ofproto: Always set MTU for new internal ports.

ofproto: Always set MTU for new internal ports.

We only change the MTU of new internal ports if it is bigger than the
bridge minimum.  But when the minimum MTU of the bridge is updated we
change the MTU of all internal ports no matter what.

The behavior is inconsistent, because now the internal ports MTU depends
on the order in which the ports were added.

This commit fixes the problem by _always_ setting the MTU of new
internal ports to the bridge minimum.  I'm not sure what was the logic
behind only adjusting the mtu if it was too big.

A testcase is improved to detect the problem.

VMware-BZ: #1718776
Signed-off-by: Daniele Di Proietto <diproiettod@vmware.com>
Acked-by: Ben Pfaff <blp@ovn.org>