mixing scenarios for dvmc exercise

- include mixing scenarios E8TeV_2012_run198588_BX_50ns 8TeV_2012_run203002_BX_50ns E8TeV_2012_run209148_BX_25ns E8TeV_2012_Zmum
- they're necessary to produce the MC samples for the data.vs.mc validation of 7_1_0, see https://twiki.cern.ch/twiki/bin/view/CMS/PdmVDataMCvalidationJuly14 
- these 4 PU scenarios have been already included in 6_2_11 and were widely used in Fall 13, but alas not fw ported to the 70-71 cycles
- I've run two tests jobs, w/o problems