Make DQMRootOutputModule a one module

Since ROOT output is thread-safe when a TFile is only being used
on one thread, we can make the DQMRootOutputModule a one module
so it does not stall any legacy modules. The PoolOutputModule is
already a one module.
Merge pull request #3941 from Dr15Jones/makeDQMRootOutputModuleOneModule

DQM -- Make DQMRootOutputModule a one module