Add options to resolve the MRs aliases in the main job

• add a set of functions to expand MR aliases (like all, label=lhcb-head or mr_iid) to a list of pairs (mr_iid, commit_id).
• add --resolve-mrs option to lbn-check-config and lbn-enabled-slots (main script of Jenkins main nightly builds job)
• the logic used to resolve MR aliases is that described in LBCORE-1819 (so using this MR will also close that issue)

Note, includes !229 (merged)

See LBCORE-1712