cardiff: Saving group differences in diff files

When specifing the '-o <dir>' option to cardiff, cardiff will save the
differences between groups in the selected directory.

It will create in it :
- ".def" files which represent the hardware configuration of a given group of servers represented by their serial number
- ".diff" files computing the differences between those groups

This will be an helper to ease the understanding between groups for a
given component.

A typical output file looks like :

Logical_Disks_C8250FD55M802ZZ.def
Logical_Disks_C8250FD55M802FF_C8250FD22M802RR_C8250FD55M802JJ.def
Logical_Disks.diff

... and the diff file looks like .. :

--- plop/Logical_Disks_C8250FD55M802FF_C8250FD55M802RR_C8250FD55M802JJ.def      2014-10-30 10:02:18.668475862 +0100
+++ plop/Logical_Disks_C8250FD55M802ZZ.def      2014-10-30 10:02:18.671475892 +0100
@@ -1 +1,8 @@
-[]
+[('disk', 'sda', 'Read Cache Disable', '0'),
+ ('disk', 'sda', 'Write Cache Enable', '1'),
+ ('disk', 'sda', 'model', 'afakedevice'),
+ ('disk', 'sda', 'optimal_io_size', '0'),
+ ('disk', 'sda', 'physical_block_size', '512'),
+ ('disk', 'sda', 'rev', '3232'),
+ ('disk', 'sda', 'size', '32'),
+ ('disk', 'sda', 'vendor', 'ATA')]