optional removed from pool_arn attribute and name changed to arn

[sumitmenaria]

data_source_ec2_coip_pool.go
pool_arn -> arn
pool_arn : removed from optional

Output from acceptance testing:

]$ make testacc TEST=./aws TESTARGS='-run=TestAccDataSourceAwsEc2CoipPool_Id'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./aws -v -count 1 -parallel 20 -run=TestAccDataSourceAwsEc2CoipPool_Id -timeout 120m
=== RUN   TestAccDataSourceAwsEc2CoipPool_Id
=== PAUSE TestAccDataSourceAwsEc2CoipPool_Id
=== CONT  TestAccDataSourceAwsEc2CoipPool_Id
--- PASS: TestAccDataSourceAwsEc2CoipPool_Id (34.08s)
PASS
ok  	github.com/terraform-providers/terraform-provider-aws/aws	34.143s

...