Attempt to fix IndexError in Opower

[tronikos]

Breaking change

Proposed change

Attempt to fix #123208
Line 113 calls get_last_statistics on cost_statistic_id. Under normal conditions the start time should match between cost and consumption statistics. My guess is that the first async_add_external_statistics call failed which caused IndexError at the next retry. Now we call get_last_statistics on the last inserted statistic, i.e. consumption instead of cost.

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[joostlek]

Should we have this in the beta?