[2.0 spring cleaning] Remove the EMR connection type.

[Ash Berlin-Taylor]

We have an EMR connection type, but the operator actually uses this as a config 
value, and the actual credentials come form the default aws_conn_id:

    def __init__(
            self,
            aws_conn_id='s3_default',
            emr_conn_id='emr_default',
            job_flow_overrides=None,
            region_name=None,
            *args, **kwargs):

Oh also: that _should_ not say 's3_default' anymore :D

I would like to propose then that we remove the emr_default conneciton, and any 
reference to a connection in the EMR* Operators, and instead change the EMR 
config to come from a Variables instead.

-ash