[process_checker] Fix 'NoSuchProcess' exception in process_checker

[bingwang-ms]

- Why I did it
The psutil library used in process_checker create a cache for each process when calling process_iter. So, there is a small possibility that one process exists when calling process_iter, but not exists when calling cmdline, which will raise a NoSuchProcess exception.

File "/usr/bin/process_checker", line 57, in <module>
    main()
  File "/usr/bin/process_checker", line 53, in main
    check_process_existence(args.container_name, ' '.join(args.process_cmdline))
  File "/usr/bin/process_checker", line 31, in check_process_existence
    if ((' '.join(process.cmdline())).startswith(process_cmdline) and process.status() in ["running", "sleeping"]):
  File "/usr/local/lib/python2.7/dist-packages/psutil/__init__.py", line 694, in cmdline
    return self._proc.cmdline()
  File "/usr/local/lib/python2.7/dist-packages/psutil/_pslinux.py", line 1522, in wrapper
    raise NoSuchProcess(self.pid, self._name)
psutil.NoSuchProcess: psutil.NoSuchProcess process no longer exists (pid=7019)

This commit fix the issue.

Signed-off-by: bingwang [email protected]

- How I did it
Add a logic to catch NoSuchProcess and then ignore it.

- How to verify it
Write a sample test script which looply creates a processe that runs only for a few milliseconds. Meanwhile, process_checker is running looply.
Without this patch, the NoSuchProcess was found, and no exception was raises after the patch.

- Which release branch to backport (provide reason below if selected)

• 201811
• 201911
• 202006

- Description for the changelog

Fix NoSuchProcess exception in process_checker

- A picture of a cute animal (not mandatory but encouraged)

[keboliu]

also see this issue in our test, hope can be cherry-picked to 201911 soon.

[jleveque]

@abdosi for visibility