Fixes#22993.
The change is only made for the minion process because, theoretically,
only the minion could create the problem described. salt-master and
salt-syndic do not theoretically spawn non-salt processes during the
lifetime of their processes, whereas salt-minion does this by design.
The default behavior for systemd killMode seems to be control-group,
which means all processes that share the same control group as the
minion process will also be killed by systemd when the minion service is
stopped (killed).
It is reasonable to expect that activity done on a system by a salt
minion should persist beyond the lifetime of the minion process, so
let's not kill procs that the minion starts even when the minion exits.
```
07:39:05,449 [salt.minion :1178][ERROR ] A command in 'pip.list' had a problem: Traceback (most recent call last):
File "/tmp/salt-tests-tmpdir/6833-pip-upgrade-pip/bin/pip", line 9, in <module>
load_entry_point('pip==1.3.1', 'console_scripts', 'pip')()
File "/tmp/salt-tests-tmpdir/6833-pip-upgrade-pip/lib/python2.6/site-packages/pkg_resources/__init__.py", line 546, in load_entry_point
return get_distribution(dist).load_entry_point(group, name)
File "/tmp/salt-tests-tmpdir/6833-pip-upgrade-pip/lib/python2.6/site-packages/pkg_resources/__init__.py", line 2666, in load_entry_point
return ep.load()
File "/tmp/salt-tests-tmpdir/6833-pip-upgrade-pip/lib/python2.6/site-packages/pkg_resources/__init__.py", line 2339, in load
return self.resolve()
File "/tmp/salt-tests-tmpdir/6833-pip-upgrade-pip/lib/python2.6/site-packages/pkg_resources/__init__.py", line 2345, in resolve
module = __import__(self.module_name, fromlist=['__name__'], level=0)
File "/tmp/salt-tests-tmpdir/6833-pip-upgrade-pip/lib/python2.6/site-packages/pip/__init__.py", line 13, in <module>
from pip.commands import commands, get_similar_commands, get_summaries
File "/tmp/salt-tests-tmpdir/6833-pip-upgrade-pip/lib/python2.6/site-packages/pip/commands/__init__.py", line 6, in <module>
from pip.commands.bundle import BundleCommand
File "/tmp/salt-tests-tmpdir/6833-pip-upgrade-pip/lib/python2.6/site-packages/pip/commands/bundle.py", line 5, in <module>
from pip.commands.install import InstallCommand
File "/tmp/salt-tests-tmpdir/6833-pip-upgrade-pip/lib/python2.6/site-packages/pip/commands/install.py", line 5, in <module>
from pip.req import InstallRequirement, RequirementSet, parse_requirements
File "/tmp/salt-tests-tmpdir/6833-pip-upgrade-pip/lib/python2.6/site-packages/pip/req/__init__.py", line 3, in <module>
from .req_install import InstallRequirement
File "/tmp/salt-tests-tmpdir/6833-pip-upgrade-pip/lib/python2.6/site-packages/pip/req/req_install.py", line 20, in <module>
import pip.wheel
File "/tmp/salt-tests-tmpdir/6833-pip-upgrade-pip/lib/python2.6/site-packages/pip/wheel.py", line 23, in <module>
from pip.exceptions import InvalidWheelFilename, UnsupportedWheel
ImportError: cannot import name InvalidWheelFilename
```
