Hlt gmbe

Integrate in ConfDB and in the HLT menus in CMSSW the change from KT6 rho correction to Grid-based median estimator (GBME) rho correction.
Besides the HLT configurations themselves this pull request includes the minimal subset of PR #1804 (only one plugin, FixedGridRhoProducerFastjet) so that it is possible to merge it standalone, without waiting for the latest fixes and following approval of 1804.