gitlab-ci: fix master pipeline

Using "when: manual" in rules causes the overall pipeline status to be "blocked" instead of "success". This has been reported as a bug in GitLab but then declared as intended behaviour 1. Suggested workaround is to add "allow_failure". Even if that's not exactly what we want, we do this for now to get the master pipeline working.

Furthermore we need the complete project path, and not just the project name, to get the correct manifest project for finding the pipeline.

Change-Id: I3ad49b435571610c716cc5cef7d98b964e7f7a32