Update default LHCbCond tag for 2018

Marco Cattaneorequested to merge
cherry-pick-f98339ee into run2-patches

Updates default LHCbCond tag for 2018 to cond-20181204-1, lhcb-conddb/LHCBCOND!29 (merged) (Reduce lumi thresholds to minimum for 2018 heavy ions)

Cherry-picked from !1656 (merged) in 2018-patches

Edited by Marco Cattaneo

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