Substituting the check_graphite script.

As I was finding it extremely difficult to interact with the original
disquis plugin I decided to spend some time to refactor it.
It is now more modular and will be easier to use and to modify according
to our purposes. The current version uses urllib2 as it's shipped with precise.

I also added two defines:
- monitor_graphite_threshold
- monitor_graphite_anomaly

that should make it easy to anyone to create checks on icinga that
originate from graphite-collected data.

Also changed the commands in check_commands.erb accordingly.

Change-Id: Ief0c4e6e50eda99cf7f94d8c5674d5573a8fc895
Signed-off-by: Giuseppe Lavagetto <glavagetto@wikimedia.org>