Add support for preemptible instances

[sina chavoshi]

preemptible instance have a significantly lower cost, which makes the perfectly suitable for individual users and experimental projects.

This issue has been raised by users.

[Fernand Pajot]

+1 ! This is why we're not considering TF Cloud yet at our company.

[Josyula Ravi Teja]

Please assign me this issue. I want to work in this issue. @fernand @SinaChavoshi @lamberta @mihaimaruseac

[Mihai Maruseac]

We don't generally assign issue to contributors. Instead, please send a PR when ready.