ofproto: Honor mtu_request even for internal ports.

ofproto: Honor mtu_request even for internal ports.

By default Open vSwitch tries to configure internal interfaces MTU to
match the bridge minimum, overriding any attempt by the user to
configure it through standard system tools, or the database.

While this works in many simple cases (there are probably many users
that rely on this) it may create problems for more advanced use cases
(like any overlay networks).

This commit allows the user to override the default behavior by
providing an explict MTU in the mtu_request column in the Interface
table.

This means that Open vSwitch will now treat differently database MTU
requests from standard system tools MTU requests (coming from `ip link`
or `ifconfig`), but this seems the best way to remain compatible with
old users while providing a more powerful interface.

Suggested-by: Darrell Ball <dlu998@gmail.com>
Signed-off-by: Daniele Di Proietto <diproiettod@vmware.com>
Acked-by: Ben Pfaff <blp@ovn.org>
Tested-by: Joe Stringer <joe@ovn.org>