Add waiter config and fix config lookup

There were two issues:

1. The emr waiter config was not there, I've now added that.
2. We were using the endpoint prefix instead of the service_name (elasticmapreduce vs. emr).

We need to use the service_name and not the endpoint prefix.
These aren't always the same thing (emr vs. elasticmapreduce).

Also added some integ tests to ensure we don't regress on this again.

Fixes #937.

cc @danielgtaylor @kyleknap