Description of problem:

When trying to run missing executable, vdsm expect to get the following error:

>>> subprocess.check_call(["foobar"])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib64/python2.7/subprocess.py", line 535, in check_call
    retcode = call(*popenargs, **kwargs)
  File "/usr/lib64/python2.7/subprocess.py", line 522, in call
    return Popen(*popenargs, **kwargs).wait()
  File "/usr/lib64/python2.7/subprocess.py", line 710, in __init__
    errread, errwrite)
  File "/usr/lib64/python2.7/subprocess.py", line 1335, in _execute_child
    raise child_exception
OSError: [Errno 2] No such file or directory

This error is typically not handled and will fail the current flow, showing
a very clear traceback.

However when vdsm is configured to use cpu_affinity, it run *all* commands
via taskset, which fail with exit code 1:

>>> subprocess.check_call(["taskset", "-c", "1", "foobar"])
taskset: failed to execute foobar: No such file or directory
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib64/python2.7/subprocess.py", line 540, in check_call
    raise CalledProcessError(retcode, cmd)
subprocess.CalledProcessError: Command '['taskset', '-c', '1', 'foobar']' returned non-zero exit status 1

In Vdsm log, we see a failure from the taskset command:

Storage.Misc.excCmd: DEBUG: /usr/bin/taskset --cpu-list 0-1 /sbin/fuser /tmp/tmpwTY0g3 (cwd None)
Storage.Misc.excCmd: DEBUG: FAILED: <err> = 'taskset: failed to execute /sbin/fuser: No such file or directory\n'; <rc> = 1

But vdsm code running the command is not aware that taskset is being
used, and consider the error as the actual command error. In the case
of fuser, exit code 1 means the file is not being opened by anyone.
This may lead to catastrophic results.

When running commands via the shell, it fail using a special exit code
127:

>>> subprocess.check_call(["sh", "-c", "foobar"])
sh: foobar: command not found
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib64/python2.7/subprocess.py", line 540, in check_call
    raise CalledProcessError(retcode, cmd)
subprocess.CalledProcessError: Command '['sh', '-c', 'foobar']' returned non-zero exit status 127

Similar tools running other processes are tyring to simulate the shell
behavior when an executable is not found.

>>> subprocess.check_call(["nice", "-n", "5", "foobar"])
nice: foobar: No such file or directory
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib64/python2.7/subprocess.py", line 540, in check_call
    raise CalledProcessError(retcode, cmd)
subprocess.CalledProcessError: Command '['nice', '-n', '5', 'foobar']' returned non-zero exit status 127

See http://tldp.org/LDP/abs/html/exitcodes.html for common special exit codes.

We tow issues:

1. execCmd is wrapping calls using nice, taskset and other wrappers, but it
   does not perform any checks on exit code, detecting failure of the wrapper
   commands, returning invalid output to the caller.

2. taskset is not reporting missing executable in a helpful way like other
   tools (e.g. nice).

Version-Release number of selected component (if applicable):
- 4.16.31 when cpu_affinity is enabled
- 4.17 and later

How reproducible:
Always

Steps to Reproduce:
1. Run vdsm tests on a system without "fuser" or "which" commands

Actual results:
Clear OSError about missing executable

Expected results:
False results (.e.g. fuser output is "")

Additional info:

If vdsm dependencies are correct, it should not be possible to remove a command
vdsm depends on.