Executor: Add support for HTTP boot

We currently only support TFTP boot to download iPXE. Let's add support for HTTP boot by creating an endpoint in app.py (/boot/bootloaders/<filename>?) that would call a new method in BootService (BootService.http_boot_query?) which would return the same files as found in the TFTP root. This should be enough to boot HTTP hosts.

The harder part will be to add integration tests for this in vivian.