benchmarks: fix benchmark.py to check MOOSE_SPEED_DB environment var
benchmarks: fix benchmark.py to check MOOSE_SPEED_DB environment var

ref #9834

dschwen/moose,milljm/moose,jessecarterMOOSE/moose,nuclear-wizard/moose,permcody/moose,jessecarterMOOSE/moose,friedmud/moose,harterj/moose,milljm/moose,andrsd/moose,permcody/moose,YaqiWang/moose,laagesen/moose,YaqiWang/moose,lindsayad/moose,milljm/moose,nuclear-wizard/moose,bwspenc/moose,harterj/moose,jessecarterMOOSE/moose,SudiptaBiswas/moose,SudiptaBiswas/moose,YaqiWang/moose,lindsayad/moose,idaholab/moose,lindsayad/moose,nuclear-wizard/moose,andrsd/moose,SudiptaBiswas/moose,bwspenc/moose,sapitts/moose,nuclear-wizard/moose,idaholab/moose,friedmud/moose,laagesen/moose,jessecarterMOOSE/moose,dschwen/moose,sapitts/moose,sapitts/moose,andrsd/moose,sapitts/moose,lindsayad/moose,idaholab/moose,harterj/moose,milljm/moose,harterj/moose,laagesen/moose,jessecarterMOOSE/moose,dschwen/moose,friedmud/moose,dschwen/moose,SudiptaBiswas/moose,lindsayad/moose,idaholab/moose,andrsd/moose,sapitts/moose,friedmud/moose,YaqiWang/moose,idaholab/moose,bwspenc/moose,SudiptaBiswas/moose,bwspenc/moose,harterj/moose,laagesen/moose,dschwen/moose,bwspenc/moose,milljm/moose,andrsd/moose,permcody/moose,laagesen/moose,permcody/moose