Fixes#9895
The replication_group_id should allow length to be max of 20 not 16
```
% make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestResourceAWSElastiCacheReplicationGroupIdValidation'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2016/11/07 16:17:52 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v
-run=TestResourceAWSElastiCacheReplicationGroupIdValidation -timeout
120m
=== RUN TestResourceAWSElastiCacheReplicationGroupIdValidation
--- PASS: TestResourceAWSElastiCacheReplicationGroupIdValidation (0.00s)
PASS
ok github.com/hashicorp/terraform/builtin/providers/aws0.032s
```
38cd7947b6