tests/data-source/aws_elasticache_cluster: Remove optional parameter_group_name argument from resource in test configuration
tests/data-source/aws_elasticache_cluster: Remove optional parameter_group_name argument from resource in test configuration

memcached 1.5 support was released and Elasticache defaults to the newer engine version, however the resource was not setting the engine version so it would previously fail with:

```
--- FAIL: TestAccAWSDataElasticacheCluster_basic (8.77s)
    testing.go:538: Step 0 error: Error applying: 1 error occurred:
        	* aws_elasticache_cluster.bar: 1 error occurred:
        	* aws_elasticache_cluster.bar: error creating Elasticache Cache Cluster: InvalidParameterCombination: Expected a parameter group of family memcached1.5 but found one of family memcached1.4
        	status code: 400, request id: e0f65211-fd45-11e8-8f41-7b9e77cae090
```

This change simply removes the optional `parameter_group_name` argument from the test configuration so it cannot be out of sync based on engine version.

Output from acceptance testing:

```
--- PASS: TestAccAWSDataElasticacheCluster_basic (712.89s)
```

Ninir/terraform-provider-aws,kjmkznr/terraform-provider-aws,Ninir/terraform-provider-aws,kjmkznr/terraform-provider-aws,terraform-providers/terraform-provider-aws,Ninir/terraform-provider-aws,kjmkznr/terraform-provider-aws,Ninir/terraform-provider-aws,terraform-providers/terraform-provider-aws,terraform-providers/terraform-provider-aws,kjmkznr/terraform-provider-aws,terraform-providers/terraform-provider-aws