WIP: Add support for resolving requisite repo references

Add support for loading a global configuration YAML file containing a map of trigger variables pointing to pipeline requisites in Git repos, such as "kpet_pip_url" or "rhel_tools_targz_url". Resolve the configured references to specific commit hashes and supply the pipeline with values pointing to them, instead of the original references (such as "master" branches).

This would let pipelines use consistent requisite revisions, and preserve them to be able to run the same versions of the requisites on retriggering, regardless of their repositories changing in the mean time.

A sample global configuration could be seen in pipeline-trigger-data MR 99.