move cron command to class parameter

Hello!

I've made a simple fix for customizing puppet run command for cron configuration.
We are still using Ubuntu 12.04 and we need to install python packages via pip. But pip has pesky bug, ignoring proxy settings other, than managed via envitonment variables. So, for our nodes I run puppet this way:
```yaml
puppet::agent::puppet_run_command: '/bin/sh -c "export http_proxy=http://proxy.example.com:3128 && export https_proxy=proxy.example.com:3128 && /usr/bin/puppet agent --no-daemonize --onetime --logdest syslog > /dev/null 2>&1"'
```

Also, this way anyone can customize agent parameters, redirections and other stuff.