Clever Runner: cache instance type query

Currently, a GET /products/instances is issued each time there is a job to provision, to find about instance types and their versions.

It would be better to cache the results on the client side (hence in heptapod-paas-runner), but that would require a way to invalidate. Typically, a change would be about the instance version and we need to use the version in that case.

For proper invalidation, we would hence need a way to detect that a provision failure is due to an outdated version, and only then invalidate the instance types and retry.