Instead construct the string using psutil.Process.cmdline() method that
will parse the data from kernel for us and will return a proper
space-separated command line (plus will provide some more safety
guarantees, e.g. against zombies).
This change also introduces a new psutil dependency for neutron. Before
that, the library was used in functional tests only, and with a minimal
version number that is not compatible with global-requirements.txt or
.cmdline(). Moved the requirement into runtime requirements.
Related-Bug: #1672921
Change-Id: I780ea10bd1870c86c1a048071863843bc2993958