Resolve "CTA CI pipeline triggered by EOS can not test against latest CTA tag in case of bugs/changes in pipeline code"

Summary

• Introduction of a standard to handle trigger pipelines:

  • All trigger related variables must be declared in the .gitlab-ci top level file. Every trigger must specify the TRIGGER_TYPE variable, along with any other required "sub"-variables.
  • The prepare stage has been renamed to TBD. It will now be the entry point for trigger pipelines. Any trigger pipeline must FIRST validate its input variable in the trigger-variable-check job. Any additional jobs to check, i.e, the modification of the version lock in EOS tag trigger pipelines.

• For this specific case of the trigger pipeline we validate the specified RPM versions are downloadable before going forward.

Additional Required Actions

• Requires a documentation update: YES, public docs need to be updated to reflect the changes and behaviour we expect for the future of trigger pipelines.
• Modify the EOS repository gitlab job to use the new flags.

References

Closes #891