[Run2_2017] Update the CMSSW version

Merged Alexx Perloff requested to merge github/fork/aperloff/Run2_2017_UpdateSetup into Run2_2017

Created by: aperloff

Change the CMSSW version to CMSSW_10_2_21. This change works as confirmed by a fresh checkout of the software and two unit tests.

Merge request reports