[sub intf] Inherit parent port mtu changes

[wendani]

What I did

Why I did it
Sub interface inherits mtu from parent physical port or port channel according to https://github.com/Azure/SONiC/blob/master/doc/subport/sonic-sub-port-intf-hld.md#1-requirements

This inheritance should include the scenario of mtu changes on parent physical port or port channel.

How I verified it
In newly added vs unit test on both master and 201911 branches:
Before the change:

E           AssertionError: Expected field/value pairs not found: expected={'SAI_ROUTER_INTERFACE_ATTR_MTU': '8888'},                 received={'SAI_ROUTER_INTERFACE_ATTR_VIRTUAL_ROUTER_ID': 'oid:0x3000000000022', 'SAI_ROUTER_INTERFACE_ATTR_SRC_MAC_ADDRESS': '02:42:AC:11:00:05', 'SAI_ROUTER_INTERFACE_ATTR_TYPE': 'SAI_ROUTER_INTERFACE_TYPE_SUB_PORT', 'SAI_ROUTER_INTERFACE_ATTR_PORT_ID': 'oid:0x1000000000012', 'SAI_ROUTER_INTERFACE_ATTR_OUTER_VLAN_ID': '10', 'SAI_ROUTER_INTERFACE_ATTR_ADMIN_V4_STATE': 'true', 'SAI_ROUTER_INTERFACE_ATTR_ADMIN_V6_STATE': 'true', 'SAI_ROUTER_INTERFACE_ATTR_MTU': '9100', 'SAI_ROUTER_INTERFACE_ATTR_NAT_ZONE_ID': '0'}, key="oid:0x60000000005c6", table="ASIC_STATE:SAI_OBJECT_TYPE_ROUTER_INTERFACE"

After the change:

test_sub_port_intf.py::TestSubPortIntf::test_sub_port_intf_mtu remove extra link dummy
PASSED

Details if related