Refactor Jenkins Jobs to Allow dedicated QEMU node

[geissonator]

QEMU runs well on some nodes, and not well on others. Need to refactor our jenkins jobs so we can dedicate a node to QEMU runs.

This will require a specific job for builds, that provides the built image via it's artifacts, and a second job that can take as input an artifact link to retrieve and use the image in a QEMU run.

[geissonator]

Here's the design:

-meta
openbmc-build
run-qemu
-openbmc
openbmc-build
run-qemu

openbmc-build is a generic job that just builds the input target/code and stores the artifacts.
run-qemu downloads the input artifact and executes qemu against it

Need to apply workaround documented in gmacario/easy-jenkins#180 (comment) to replace the @ symbol with the - symbol for concurrent builds to work properly in openbmc-build.

[geissonator]

Done within https://openpower.xyz/view/CI/ jobs.