ci(feat): allow to select what job to launch using labels

This allow to set a label on a MR that has the name of one of the python jobs like "py3-server-core".

If one or more labels are set: only py3-* jobs match the labels will run

If not label is set: all py3 jobs will run.

This allows to lower the load on some MRs when a lot of work needs to be done but only a few jobs are interesting (like "py3-server-core" for slapd)