Executor: Add caching ability of kernels/initramfs'

In order to save on internet bandwidth and reduce our reliance on external services being available, we should start caching the kernels and initramfs and re-use them as much as possible.

We may want to add a new parameter in the job description to enable such feature, as caching solely based on the URL would break jobs that rely on a fixed URL that gets updated. Another option would be to keep in the cache the information about the last update and use the HTTP headers to decide whether to cache the file or not, and for this we could simply use an HTTP client that has local caching.