FIX: Fixed a bug on single service override attribute.

When a single `service_overrides` attribute is set on a host, it is splitted on comma used as host/service delimiter, and attribute value get false.

This patch adds an additional property attribute named `split_on_coma` that indicates if a single value should be splitted on coma delimiter.

The `service_overrides` attribute is explicitely declared as non splitted.