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*. In some situations, it allows to write business rules in a more natural way.

Example:

A business rule using an hostgroup composed of `10` hosts in which only `1` is authorized to fail may be written `9 of: g:hostghoup`, or `-1 of: g:hostghoup` which is exactly equivalent. If the hostgroup grows, there is still only one failure authorized in the second form, the first needs to be manually updated.

Also factorised `Xof` elements processing using an inner function. Finally, fixed `test_properties_defaults.py` (forgot to update it in previous pull request).