provider/aws: Add support for treat_missing_data to cloudwatch_metric_alarm (#13358)
provider/aws: Add support for treat_missing_data to cloudwatch_metric_alarm (#13358)

* provider/aws: Add support for treat_missing_data to cloudwatch_metric_alarm

Fixes: #13263

```
% make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSCloudWatchMetricAlarm'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/04/05 08:51:06 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSCloudWatchMetricAlarm -timeout 120m
=== RUN   TestAccAWSCloudWatchMetricAlarm_importBasic
--- PASS: TestAccAWSCloudWatchMetricAlarm_importBasic (23.93s)
=== RUN   TestAccAWSCloudWatchMetricAlarm_basic
--- PASS: TestAccAWSCloudWatchMetricAlarm_basic (27.81s)
=== RUN   TestAccAWSCloudWatchMetricAlarm_treatMissingData
--- PASS: TestAccAWSCloudWatchMetricAlarm_treatMissingData (43.39s)
=== RUN   TestAccAWSCloudWatchMetricAlarm_extendedStatistic
--- PASS: TestAccAWSCloudWatchMetricAlarm_extendedStatistic (26.80s)
=== RUN   TestAccAWSCloudWatchMetricAlarm_missingStatistic
--- PASS: TestAccAWSCloudWatchMetricAlarm_missingStatistic (5.95s)
PASS
ok  	github.com/hashicorp/terraform/builtin/providers/aws	127.899s
```

* provider/aws: Set cloudwatch_metric_alarm treamt_missing_data to missing

This follows what the AWS API does. We had to add a state migration for
this to make sure that the user doesn't see any unexpected activity on
their Terraform plans

```
% make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAWSCloudWatchMetricAlarmMigrateState'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/04/05 14:51:32 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAWSCloudWatchMetricAlarmMigrateState -timeout 120m
=== RUN   TestAWSCloudWatchMetricAlarmMigrateState
2017/04/05 14:52:13 [INFO] Found AWS CloudWatch Metric Alarm State v0; migrating to v1
2017/04/05 14:52:13 [DEBUG] Attributes before migration: map[string]string{}
2017/04/05 14:52:13 [DEBUG] Attributes after migration: map[string]string{"treat_missing_data":"missing"}
--- PASS: TestAWSCloudWatchMetricAlarmMigrateState (0.00s)
PASS
ok  	github.com/hashicorp/terraform/builtin/providers/aws	0.018s
```

nbaztec/terraform-provider-aws,Ninir/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,ClementCunin/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,nbaztec/terraform-provider-aws,kjmkznr/terraform-provider-aws,terraform-providers/terraform-provider-aws,kjmkznr/terraform-provider-aws,ClementCunin/terraform-provider-aws,kjmkznr/terraform-provider-aws,ashinohara/terraform-provider-aws,ClementCunin/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,ashinohara/terraform-provider-aws,Ninir/terraform-provider-aws,terraform-providers/terraform-provider-aws,ClementCunin/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,nbaztec/terraform-provider-aws,kjmkznr/terraform-provider-aws,ashinohara/terraform-provider-aws,nbaztec/terraform-provider-aws,Ninir/terraform-provider-aws,ashinohara/terraform-provider-aws,terraform-providers/terraform-provider-aws,terraform-providers/terraform-provider-aws,Ninir/terraform-provider-aws