Fix for KeyError: 'DEVICE_NEIGHBOR' when executing 'show interfaces neighbor expected' command

[ramachandrareddygaddam]

- What I did
Handle exceptions (KeyError and ValueError), when user issued 'show interfaces neighbor expected' command and
DEVICE_NEIGHBOR data is not present configuration file.

- How I did it
In the src/sonic-config-engine/sonic-cfggen file 'KeyError' exception handled, when DEVICE_NEIGHBOR or DEVICE_NEUGHBOR_METADATA information is not present.
In the sonic-utilities/show/main.py utility, 'ValueError' exception handled.

- How to verify it
Issue 'show interfaces neighbor expected' command and observe no exceptions.
If DEVICE_NEIGHBOR data present in configuration file, data will be displayed.

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

root@sonic-testing:/home/admin# show interfaces neighbor expected
Traceback (most recent call last):
File "/usr/local/bin/sonic-cfggen", line 263, in
main()
File "/usr/local/bin/sonic-cfggen", line 247, in main
print(json.dumps(FormatConverter.to_serialized(data[args.var_json]), indent=4, cls=minigraph_encoder))
KeyError: 'DEVICE_NEIGHBOR'
Traceback (most recent call last):
File "/usr/bin/show", line 9, in
load_entry_point('sonic-utilities==1.2', 'console_scripts', 'show')()
File "/usr/lib/python2.7/dist-packages/click/core.py", line 722, in call
return self.main(*args, **kwargs)
File "/usr/lib/python2.7/dist-packages/click/core.py", line 697, in main
rv = self.invoke(ctx)
File "/usr/lib/python2.7/dist-packages/click/core.py", line 1066, in invoke
return _process_result(sub_ctx.command.invoke(sub_ctx))
File "/usr/lib/python2.7/dist-packages/click/core.py", line 1066, in invoke
return _process_result(sub_ctx.command.invoke(sub_ctx))
File "/usr/lib/python2.7/dist-packages/click/core.py", line 1066, in invoke
return _process_result(sub_ctx.command.invoke(sub_ctx))
File "/usr/lib/python2.7/dist-packages/click/core.py", line 895, in invoke
return ctx.invoke(self.callback, **ctx.params)
File "/usr/lib/python2.7/dist-packages/click/core.py", line 535, in invoke
return callback(*args, **kwargs)
File "/usr/lib/python2.7/dist-packages/show/main.py", line 426, in expected
neighbor_dict = json.loads(p1.stdout.read())
File "/usr/lib/python2.7/json/init.py", line 339, in loads
return _default_decoder.decode(s)
File "/usr/lib/python2.7/json/decoder.py", line 364, in decode
obj, end = self.raw_decode(s, idx=_w(s, 0).end())
File "/usr/lib/python2.7/json/decoder.py", line 382, in raw_decode
raise ValueError("No JSON object could be decoded")
ValueError: No JSON object could be decoded
root@sonic-testing:/home/admin#

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

root@sonic-testing:/home/admin# show interfaces neighbor expected
DEVICE_NEIGHBOR information is not present.

root@sonic-testing:/home/admin#

[jleveque]

Retest this please

[ramachandrareddygaddam]

To resolve the issue completely, the below fix also needed
sonic-net/sonic-buildimage#2535