Safe reboots

Add commands to safely reboot machines which are:

 * [safe to reboot](https://github.gds/pages/gds/opsmanual/2nd-line/rebooting-machines.html)
 * not a datastore such as mongo or elasticsearch
 * not a monitoring box which will cause alert confusion if rebooted
 * have the same number in their group
As this is a new file, it's going to need listing in [fabfile.py](https://github.com/alphagov/fabric-scripts/blob/master/fabfile.py#L16-L17)