AthenaMonitoringKernel: Fix setting of histogram options.

When histogram options were provided via a list of strings to the opt argument of defineHistogram(), only the last option actually had any effect. This is because lists were being handled with this code:

   elif isinstance(opt, list):
       for o in opt: settings.update( _options(o) ) # process each item in list

but _options() returns all the settings, so this was overwriting all but the last option with defaults.

Reworked to fix this.

Fixes assertion failure seen in the TrigT1CaloMonitoring in the dbg build.