Add IPsec VTI interface MTU support
Currently, IPsec VTI interfaces have no special handling for MTU. It is possible to nudge it manually after the system boots, but the settings are not retained/applied at boot time.
Set IPsec VTI MTU to configured value at boot. Implements #9111
(cherry picked from commit 3334f9c4cd7111c624ba2395b91c065d7dd338b1)