V2 API Support for before_end param configuration

Adding the possibility to modify the before_end_lease event
through the REST API by adding a new parameter on the JSON request called
"before_end_notification".
The change was implemented for v2 API version and it affects both
create lease and update lease operations

Implements: blueprint before-end-notification-crud
Change-Id: Ia02b2481d33efb60771975f0181d091499594ccf