MinIO/Executor: Un-hardcode bucket provisioning and the default kernels/initrd/cmdline

Up until now, we have had a hardcoded configuration/provisioning of the minio buckets (minio_configure.sh.j2) and a single default kernel/initrd/cmdline.

This series addresses these shortcomings by moving the provisioning of minio to gateway.yml (which can be overloaded by other users of the infra), and making the executor source the default boot configuration from environment variables set by the minio_configure.sh script at runtime (based on gateway.yml).

This will allow us to boot non-x86_64 targets very soon :)

Depends on !525 (merged)