support a connect string passed with global tag

Requires changes in StdBase and PromptReco. Should be transparent if no connect string is used.

@ticoann , please review, especially the changes made to StdBase. I added an optional GlobalTag and GlobalTagConnect parameter, as they are referenced on the code (so they should also be defined there). They are optional though, other specs can re-declare them as mandatory (as they already should as the GlobalTag parameter wasn't previously define in StdBase at all).

PS: This is deployed in the production Tier0 since an hour ago.