icinga: replace naggen

Given our actual naggen would have a lot of problems with puppet 3, we
decided to substitute it with a script written from scratch in python,
aiming mainly at a much better speed of creation. Preliminary tests show
improvements of ~ 1 order of magnitude for execution times.

Change-Id: I5a0751eede3cf830ed653cfef77c99088e408b98
Signed-off-by: Giuseppe Lavagetto <glavagetto@wikimedia.org>