Add nagios.schedule_downtime command

Adds the ability to schedule downtime for a host in nagios. You call it
like this:

    fab staging nagios.schedule_downtime:backend-3.backend.production,5

for 5 minutes of downtime on backend-3.backend.production.
Merge pull request #16 from alphagov/add-nagios-downtime-command

Add nagios.schedule_downtime command