Add proper retry config for emr

Two issues:

* We need to use the proper endpoint prefix of "elasticmapreduce",
  not "emr".
* We need to use throttling_exception instead of the throttling
  rule for retry.
Change retry base for emr to be 2.1

Based on guidance from the EMR team.
Merge branch 'emr-retry-json' into develop

* emr-retry-json:
  Change retry base for emr to be 2.1
  Add proper retry config for emr
Add default values of metadata timeout/num_attempts

Fixes broken build, as None values were being passed through
to the fetcher class.