remove minlink initial value while fallback is define

[twtseng-tim]

- What I did
remove minlink initial value while fallback is define [#2053]
- How I did it

- How to verify it
add <Fallback>true</Fallback> in minigraph.xml where defined Portchannel
Do: config load_minigraph -y

- Description for the changelog

before this change I got following error message:

Running command: /usr/local/bin/sonic-cfggen -H -m -j /etc/sonic/init_cfg.json--write-to-db
Traceback (most recent call last):
  File "/usr/local/bin/sonic-cfggen", line 280, in <module>
    main()
  File "/usr/local/bin/sonic-cfggen", line 212, in main
    deep_update(data, parse_xml(minigraph, platform))
  File "/usr/local/lib/python2.7/dist-packages/minigraph.py", line 562, in parse_xml
    (intfs, lo_intfs, mgmt_intf, vlans, vlan_members, pcs, pc_members, acls, vni) = parse_dpg(child, hostname)
  File "/usr/local/lib/python2.7/dist-packages/minigraph.py", line 206, in parse_dpg
    pcs[pcintfname] = {'members': pcmbr_list, 'fallback': pcintf.find(str(QName(ns, "Fallback"))).text, 'min_links': str(int(math.ceil(len() * 0.75)))}
TypeError: len() takes exactly one argument (0 given)

after this change it can load normally:

~$ sudo config load_minigraph -y
Running command: systemctl stop dhcp_relay
Running command: systemctl stop swss
Running command: systemctl stop snmp
Warning: Stopping snmp.service, but it can still be activated by:
  snmp.timer
Running command: systemctl stop lldp
Running command: systemctl stop pmon
Running command: systemctl stop bgp
Running command: systemctl stop teamd
Running command: systemctl stop hostcfgd
Running command: /usr/local/bin/sonic-cfggen -H -m -j /etc/sonic/init_cfg.json --write-to-db
Running command: pfcwd start_default
Running command: config qos reload
Buffer definition template not found at /usr/share/sonic/device/x86_64-accton_as7816_64x-r0/Accton-AS7816-64X/buffers.json.j2

Running command: systemctl restart hostname-config
Running command: systemctl restart interfaces-config
Running command: systemctl restart ntp-config
Running command: systemctl restart rsyslog-config
Running command: systemctl restart swss
Running command: systemctl restart bgp
Running command: systemctl restart teamd
Running command: systemctl restart pmon
Running command: systemctl restart lldp
Running command: systemctl restart snmp
Running command: systemctl restart dhcp_relay
Running command: systemctl restart hostcfgd
Please note setting loaded from minigraph will be lost after system reboot. To preserve setting, run `config save`.
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[twtseng-tim]

Hi, @hzheng5 @stcheng , should i do any modify for pass this pr.

[twtseng-tim]

Hi, @hzheng5 @stcheng , Is there any suggestion for this pr?

[pavel-shirshov]

I think we should put pcmbr_list as len parameter here.

[twtseng-tim]

I see here #2053 (comment) decided to remove this parameter
fallback support multi-member now?

[pavel-shirshov]

sonic-net/sonic-swss#951