Some improvement for Mellanox sniffer CLI

[kevinwangsk]

• Hide Mellanox sniffer CLI on other platforms
• Set enable/disable to be the argument of sniffer command rather
  than the sub-command

Signed-off-by: Kevin Wang kevinw@mellanox.com

- What I did
Make the CLI "config/show platform mlnx" just appear on Mellanox platform.
- How I did it
Use sonic_platform.get_sonic_version_info()['asic_type'] to identify the running platform.
- How to verify it
Should not see "config/show platform mlnx" CLI on the other platforms.
- Previous command output (if the output of a command-line utility has changed)

- New command output (if the output of a command-line utility has changed)

-->

[qiluo-msft]

This will break 'show interface status' or 'config' in vs. Please check.

root@346360b6760c:/# show interface status
Traceback (most recent call last):
  File "/usr/bin/show", line 9, in <module>
    load_entry_point('sonic-utilities==1.2', 'console_scripts', 'show')()
  File "/usr/local/lib/python2.7/dist-packages/pkg_resources/__init__.py", line 484, in load_entry_point
    return get_distribution(dist).load_entry_point(group, name)
  File "/usr/local/lib/python2.7/dist-packages/pkg_resources/__init__.py", line 2714, in load_entry_point
    return ep.load()
  File "/usr/local/lib/python2.7/dist-packages/pkg_resources/__init__.py", line 2332, in load
    return self.resolve()
  File "/usr/local/lib/python2.7/dist-packages/pkg_resources/__init__.py", line 2338, in resolve
    module = __import__(self.module_name, fromlist=['__name__'], level=0)
  File "/usr/lib/python2.7/dist-packages/show/main.py", line 638, in <module>
    if (sonic_platform.get_sonic_version_info()['asic_type'] == 'mellanox'):
TypeError: 'NoneType' object has no attribute '__getitem__'

root@346360b6760c:/# config ?
Traceback (most recent call last):
  File "/usr/bin/config", line 9, in <module>
    load_entry_point('sonic-utilities==1.2', 'console_scripts', 'config')()
  File "/usr/local/lib/python2.7/dist-packages/pkg_resources/__init__.py", line 484, in load_entry_point
    return get_distribution(dist).load_entry_point(group, name)
  File "/usr/local/lib/python2.7/dist-packages/pkg_resources/__init__.py", line 2714, in load_entry_point
    return ep.load()
  File "/usr/local/lib/python2.7/dist-packages/pkg_resources/__init__.py", line 2332, in load
    return self.resolve()
  File "/usr/local/lib/python2.7/dist-packages/pkg_resources/__init__.py", line 2338, in resolve
    module = __import__(self.module_name, fromlist=['__name__'], level=0)
  File "/usr/lib/python2.7/dist-packages/config/main.py", line 929, in <module>
    if (sonic_platform.get_sonic_version_info()['asic_type'] == 'mellanox'):
TypeError: 'NoneType' object has no attribute '__getitem__'

[qiluo-msft]

The quick fix is reverting.
The root cause is missing /etc/sonic/sonic_version.yml in vs docker sonic-net/sonic-buildimage#2134