ci: mkplan.py for dynamic plan generation

We need to generate plans dynamically when automating the running of
fastpath on GitLab. Do this by introducing a directory structure to hold
sut, config and benchmark fragments. A script can then be run with
parameters to stitch the fragments together.

To start with, we have a fragment for a single SUT, the Grivton3
m7g.metal instance we have for benchmarking. And we have 2 benchmark
list options; "smoke.yaml" runs a single benchamark just to prove out
the framework functionality. "full" runs all the benchmarks we have.
Over time, more can be added.

The config always comes from a separate source, and for now is just the
kernel and modules URLs.

The script locates the fragments, stitches them together and writes the
result to stdout:

  $ mkplan.py --sut m7g.metal-01 --benchmarks full --config ./mycfg.yaml

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>