Update add_utc_tz function to throw if a datetime object which is passed in already contains a non UTC timezone.
Update add_utc_tz function to throw if a datetime object which is passed in
already contains a non UTC timezone.

Not doing that means we are simply replacing tzinfo and not actually converting
datetime object to UTC timezone which means loss of information and invalid
result.

armab/st2,pixelrebel/st2,StackStorm/st2,Itxaka/st2,StackStorm/st2,dennybaa/st2,peak6/st2,grengojbo/st2,tonybaloney/st2,nzlosh/st2,lakshmi-kannan/st2,lakshmi-kannan/st2,grengojbo/st2,armab/st2,pixelrebel/st2,dennybaa/st2,emedvedev/st2,alfasin/st2,emedvedev/st2,peak6/st2,tonybaloney/st2,punalpatel/st2,punalpatel/st2,StackStorm/st2,Itxaka/st2,alfasin/st2,Itxaka/st2,Plexxi/st2,emedvedev/st2,Plexxi/st2,lakshmi-kannan/st2,grengojbo/st2,nzlosh/st2,pixelrebel/st2,tonybaloney/st2,peak6/st2,nzlosh/st2,punalpatel/st2,alfasin/st2,Plexxi/st2,Plexxi/st2,StackStorm/st2,nzlosh/st2,armab/st2,dennybaa/st2