Handle missing data in grouped timeseries

Pivotal story: https://www.pivotaltracker.com/s/projects/911874/stories/65548626

Fixed for both grouped_timeseries and grouped_timeshift; tests updated; and grouped_timeshift tests slightly refactored. 

Now both collectors will simply ignore any series that are specified in Stagecraft but do not appear in the Backdrop response, and Spotlight will not break.