[HltEfficiencyChecker]: yaml parsing of rates groups doesn't work

Note: HltEfficiencyChecker is being moved from MooreAnalysis to DaVinci in !1005 (merged). This issue only becomes relevant to DaVinci once that is merged

Discovered by @lpica - many thanks!

If you try a line like:

rates_groups: Muon:Hlt1TrackMuonMVALineDecision,Hlt1SingleHighPtMuonLineDecision MVA:Hlt1TwoTrackMVALineDecision,Hlt1TrackMVALineDecision

The yaml translation to command line args won't handle this properly. You'll get '--rates-groups=Muon:Hlt1TrackMuonMVALineDecision,Hlt1SingleHighPtMuonLineDecision MVA:Hlt1TwoTrackMVALineDecision,Hlt1TrackMVALineDecision', and I think this doesn't work properly with the nargs='+' argparsing in hlt_calculate_rates.py. There should be no equals sign.