aws_elasticache_replication_group caused a panic
Fixes#9097
The update for `parameter_group_name` was trying to find the incorrect
value to set `cache_parameter_group_name` - this is what was causing the
panic
```
% make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSElasticacheReplicationGroup_updateParameterGroup'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2016/09/28 11:17:30 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v
-run=TestAccAWSElasticacheReplicationGroup_updateParameterGroup -timeout
120m
=== RUN TestAccAWSElasticacheReplicationGroup_updateParameterGroup
--- PASS: TestAccAWSElasticacheReplicationGroup_updateParameterGroup
(903.90s)
PASS
ok github.com/hashicorp/terraform/builtin/providers/aws903.931s
```
The AWS documentation tells us the following:
```
--replication-group-id (string)
The replication group identifier. This parameter is stored as a
lowercase string.
Constraints:
A name must contain from *1 to 20* alphanumeric characters or hyphens.
The first character must be a letter.
A name cannot end with a hyphen or contain two consecutive hyphens.
```
This is not correct and is causing users errors:
```
* aws_elasticache_replication_group.bar: Error creating Elasticache
* Replication Group: InvalidParameterValue: Replication group id should
* be no more than 16 characters.
status code: 400, request id:
```
Tuning the Validation from 20 to 16 characters to avoid user issues