replace non-existing log_action() with print()

Inspired by: [Python Fabric NameError: global name 'log_action' is not defined](http://stackoverflow.com/q/22696486/4279)
Fixed the url of the contributing guide
Rephrase nohup part of FAQ
Fix bug in env.use_shell docs - run/sudo functions doesn't have use_shell argument.
Merge branch 'master' into 1106-int
Changelog, closes #1106
Merge pull request #1110 from konradhalas/master

Fix bug in env.use_shell docs
Merge pull request #1109 from humanfromearth/master

Fixed the url of the contributing guide
Clean up references to contribution-guide.org & add explicit note about changelog location
Set the reboot command

This update makes it possible to set the reboot command. In our case when we use molly-guard to prevent accidental reboots we need to use /sbin/reboot and not reboot.
Cleanup of #1101. Added changelog entry
Update docs - env.all_hosts default value

Default value for env.all_hosts is [], not None.