feature(create_test_release_jobs): Add job metadata files

[k0machi]

This commit adds a simple per-folder definition file (and format
documentation) format for storing additional metadata on the jenkins
jobs. The metadata is appended to the end of job description and is
planned to be consumed by Argus to determine both the job type and
subtype.

Fixes scylladb/argus#428

Testing

• [ ]

PR pre-checks (self review)

• I added the relevant backport labels
• I didn't leave commented-out/debugging code

Reminders

• Add New configuration option and document them (in sdcm/sct_config.py)
• Add unit tests to cover my changes (under unit-test/ folder)
• Update the Readme/doc folder relevant to this change (if needed)