Add preemptible feature

[nbttmbrg]

• Adds boolean parameter "preemptible" to be set in platforms. As Molecule instances are often short lived instances this will allow to mitigate the financial consequences of a forgoten running instance (particularly for pay-as-you-go licenced VMs, such as Windows or SLES/RHEL).
• Modifies Readme to describe the parameter

[ssbarnea]

That is nice. In fact I was considering adding a feature in molecule itself where all the resources created would have a limited lifespan of something like an hour, user overridable.

The reason is the same: limit the costs and avoid accidents where testing leftovers remain running for days, weeks,...

[nbttmbrg]

Yes, I would have even preferred being able to destroy a regular instance after a limited time instead of relying on preemptible instances, that feature would be great !