New platform.json file cause Mellanox ACS-2700 failed to get port information

[Junchao-Mellanox]

Description

After platform.json being added to device/mellanox/x86_64-mlnx_msn2700-r0, mellanox ACS-2700 cannot load minigraph due to failed to get port information. The reason could be that device_info.get_path_to_port_config_file() returns platform.json instead of port_config.ini when platform.json exists. And in the current platform.json, there is no interfaces defined. See commit f219401, section "interfaces" is empty.

Steps to reproduce the issue:

1. issue "config load_minigraph -y" on a Mellanox ACS-2700 platform.

Describe the results you received:

Warning: failed to retrieve PORT table from ConfigDB!

Describe the results you expected:

It should be able to get all port information.

Additional information you deem important (e.g. issue happens only occasionally):

**Output of `show version`:**

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

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