Bug-fixing of b-tag validation code.

Bug-fixing of b-tag validation code, using HLT_PFMET120_NoiseCleaned_BTagCSV07 instead of HLT_PFMET170.

In very few events it happened that HLT_PFMET170_NoiseCleaned_v1 pass, but no b-tag variables were reconstructed (although HLT_PFMET120_NoiseCleaned_BTagCSV07_v1 use lower threshold).
This because HLT_PFMET120_NoiseCleaned_BTagCSV07 use also a cut on "hltMETCleanUsingJetID80".

Now, I select only events that pass HLT_PFMET120_NoiseCleaned_BTagCSV07, so we have BTag variables for sure.

Silvio.