Limit CPU usage of puppet agent run

Idea: Use Linux [control groups](http://en.wikipedia.org/wiki/Cgroups)
```
# Install
apt-get install cgroup-bin
mount -t cgroup -o defaults cgroup /sys/fs/cgroup # See https://wiki.debian.org/LXC

# Create and configure cgroup
cgcreate -a root -g cpu:foo
echo 10 > /sys/fs/cgroup/foo/cpu.shares

# Run command
cgexec -g *:foo ls -l
```

On VM the last command fails with:
```
cgroup change of group failed
```

@cm-jan maybe you wanna help/investigate?

Research:
- http://forums.fedoraforum.org/showthread.php?t=286332
- https://wiki.debian.org/LXC