Added reversed (negative) xof business rules.

Standard Xof: business rules are used to express idioms such as "at
least X services OK", or "at least X hosts UP". Negative business rules allow
to use the reversed form to express idioms such as "at most X services
WARNING or CRITICAL" or "at most X hosts DOWN".

Also factorised Xof elements processing using an inner function.

Finall, fixed test_properties_defaults.py (forgot to update it in
previous pull request).
Merge pull request #992 from geektophe/pb_rules_negative_xof

Added reversed (negative) Xof business rules.