prefer std::variant over boost::variant

@cattanem : please unWIP / add labels / merge at your leisure... @ibelyaev : FYI

• [2018-07-21 00:11] Validation started with lhcb-gaudi-head#1937
• [2018-07-22 00:09] Validation started with lhcb-gaudi-head#1938
• [2018-07-22 00:09] Validation started with lhcb-lcg-dev3#593
• [2018-07-22 00:10] Validation started with lhcb-head#1926
• [2018-07-22 00:12] Validation started with lhcb-tdr-test#224
• [2018-07-22 00:12] Validation started with lhcb-lcg-dev4#593
• [2018-07-22 11:12] Validation started with lhcb-lcg-dev3#594
• [2018-07-23 00:07] Validation started with lhcb-head#1927
• [2018-07-23 00:08] Validation started with lhcb-lcg-dev3#595
• [2018-07-23 00:08] Validation started with lhcb-lcg-dev4#594
• [2018-07-23 00:11] Validation started with lhcb-tdr-test#225
• [2018-07-23 00:11] Validation started with lhcb-gaudi-head#1939

unmarked as a Work In Progress

merged

mentioned in commit b29ce90d