Revert "Revert "Switching default tau MVA input to ConfDB""

This reverts commit 2208d7537baf58cc52de24bb418586500a47fd31.
removed all references to root input files and moved them to auxiliary function. Protections for invalid python cfg added.
Merge pull request #6949 from jpavel/740p2_taus_switchConfDB

Switching default tau MVA input to ConfDB (attempt 2)
Merge remote branch 'origin/CMSSW_7_4_X' into CMSSW_7_0_X