Switching default tau MVA input to ConfDB (attempt 2)

Fixing the mess caused by #6776 by removing all references to input root files from CMSSW and placing them to auxiliary function. Added also the protection against possible wrong configuration input. The original description still applies, however:

Having moved the tau payloads to the production database, tau sequence is switching input for all its MVA discriminators and at the same time removing the root files from the CMSSW. The former behaviour can be restored by
1) re-including the root files
2) adding the 2 lines below to the config python to swich the input
```
from RecoTauTag.Configuration.switchMVAtoDB_cfi import switchMVAtoDB
process = switchMVAtoDB(process)
```

No changes in tau performance expected as the input should be identical.

@andrew314
@veelken