make the choice between DQM and DQMIO configurable

Make the DQM data tier configurable, can be specified as setDQMDataTier(tier0Config, <datatier>). Only "DQM" and "DQMIO" are supported, the latter is the default.
Merge pull request #4106 from hufnagel/configurable-dqm

make the choice between DQM and DQMIO configurable