Add support for reading in RDS read replicas

Add read_replica_dbinstance_identifiers as a list of the
read replicas for a given db.
Remove debug print statement
Reformat long lines
Add docstring for new DBInstance attribute
Merge branch 'katzj-rds-read-replicas' into develop

* katzj-rds-read-replicas:
  Add docstring for new DBInstance attribute
  Reformat long lines
  Remove debug print statement
  Add support for reading in RDS read replicas