aggregation differentiators regression fix

aggregate by ignoring earlier set of differentiators of earlier
aggregates.

this used to work fine, but
since dd4e8b98 tag values can be a tuple, set in
graphite_func_aggregate(), t[0] being the real value
and t[1] the list differentatiors

since then, the differentatiors were accidentially included
in the variables_str and caused subsequent aggregation
(i.e. avg_by cause sum_by is always first) rules to be more strict,
by looking at which values for the sum_by tag were aggregated

while we're at it, make the agg_keys a little clearer by using = and ,