Allow veth_mtu to be configured in openvswitch agent

With openvswitch vlan, default of veth_mtu = 1500 is 4 bytes short and you end up
with either no traffic or abysmal network performance.  Need the 4 extra bytes for
the vlan id.

Change-Id: Idc128c76f6b93ce52acc51bba9da08ccaa308ac3
Closes-Bug: #1325754