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