adding HLT_xAOD__TrigMissingETContainer_TrigEFMissingET_mht to EXOT9 and EXOT17
requested to merge ysmirnov/athena_old:21.2-AddingOnlineMetContainerToEXOT9and17-2019-10-31 into 21.2
This MR adds an additional container into EXOT9 and EXOT17 derivations. It is needed to get hold of the missing ET seen at the trigger level. Container size is 0.204 kB/event in MC, 0.258 kB/event in data. Relative increase in the file size is +0.64% for MC and +0.87% for data. Tagging @mkbugge and @guescini.