Display only external ports on single-asic systems by default

[arista-nwolfe]

Align the behavior of multi-asic and single-asic to only display external ports by default.
This will fix certain sonic-mgmt tests on single-asic systems with internal ports that don't expect them to be there.
See sonic-net/sonic-mgmt#18249

Without this sonic-mgmt function calls like:
duthost.show_interface(command="status", include_internal_intfs=False)['ansible_facts']['int_status']
Would return internal ports on single-asic systems despite being passed include_internal_intfs=False

Without this change qos/test_pfc_counters.py fails on single-asic UT2 systems with:

        conn_facts = conn_graph_facts['device_conn'].get(duthost.hostname, {})
...
        int_status = asic.show_interface(command="status")[
            'ansible_facts']['int_status']
        """ We only test active physical interfaces """
        active_phy_intfs = [intf for intf in int_status if
                            intf.startswith('Ethernet') and
                            int_status[intf]['admin_state'] == 'up' and
                            int_status[intf]['oper_state'] == 'up']
...
                for intf in active_phy_intfs:
>                   peer_device = conn_facts[intf]['peerdevice']
E                   KeyError: 'Ethernet-Rec0'

With this change the test passes.

[mssonicbld]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[arista-nwolfe]

/azpw run

[mssonicbld]

/AzurePipelines run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).