[Test Gap] test_vlan_ping doesn't work for Broadcom platform

[StormLiangMS]

Description test_vlan_ping failed on all broadcom platform

Steps to reproduce the issue:

1. Run test_vlan_ping on broadcom platform

Describe the results you received: octets = ip_str.split('.') if len(octets) != 4:

      raise AddressValueError("Expected 4 octets in %r" % ip_str)

E AddressValueError: Expected 4 octets in u'grat_arp'

cls = <class 'ipaddress.IPv4Interface'> ip_str = 'grat_arp' octets = ['grat_arp']

Describe the results you expected:

Additional information you deem important:

**Output of `show version`:**

```
(paste your output here)
```

**Attach debug file `sudo generate_dump`:**

```
(paste your output here)
```

[yxieca]

@adyeung , can Broadcom help take a look at this issue?