Nagios

Todo:
- [x] Add check for memory
- [x] Add check for disk 
- [x] Add check for apt update status
- [x] Fill in metadata for nagios-services cookbook
- [x] Allow port 5666 only for the IP of monitoring-host.
- [x] Fix ssh being blocked until second chef-client run. (so, first check-in after bootstrap)
- ~~Figure out how mail settings work and check if nagios is able to send alert mails to users~~ (out of scope for now)
- ~~Experiment with http://nagiosgraph.sourceforge.net/. Find or make cookbook that installs it.~~ (out of scope for now)
- ~~Learn more about OpenID / other authentication methods~~ (Related, but belongs in another topic)