[keen-io] Keen's scoped keys are impossible to use

[SegmentDestinationsBot]

Migrated from #2 by @ndhoule

From @d4rky-pl on April 14, 2015 12:21

When we add Keen.io integration to Segment it forces us to provide read key in admin panel rather than anywhere in the code. This makes it impossible to use Keen's scoped keys as they are generated and can be unique for example per app user.

Copied from original issue: #591