IOTlab: make destroy stateless & idempotent

Stateless:

The IOTlab provider sets an internal state when doing an init (it stores the job id). Following other's provider philosphy we could only rely on the job_name (written in the conf) to look for a current reservation and destroy it. This stateless version would allow to destroy the reservation even if the init fails before storing the job_id for instance.

Idempotent:

In the current implementation, calling destroy twice fails