resource/aws_elasticsearch_domain: Ensure in-place version upgrade is fully successful before returning (#11793)
resource/aws_elasticsearch_domain: Ensure in-place version upgrade is fully successful before returning (#11793)

Previously in the acceptance testing:

```
--- FAIL: TestAccAWSElasticSearchDomain_update_version (901.47s)
    testing.go:640: Step 2 error: Check failed: Check 3/3 error: aws_elasticsearch_domain.test: Attribute 'elasticsearch_version' expected "5.6", got "5.5"
```

The AWS Elasticsearch service upgrade process uses the same response object with multiple step values ([Elasticsearch User Guide](https://docs.aws.amazon.com/elasticsearch-service/latest/developerguide/es-version-migration.html) or more clearly, the [AWS CLI documentation](https://docs.aws.amazon.com/cli/latest/reference/es/get-upgrade-status.html)). The previous logic had race condition for when the actual upgrade step was the one that was completed, e.g.

```
2020/01/28 21:47:05 [DEBUG] [aws-sdk-go] DEBUG: Response es/GetUpgradeStatus Details:
...
2020/01/28 21:47:05 [DEBUG] [aws-sdk-go] {"StepStatus":"SUCCEEDED","UpgradeName":"Upgrade from 5.5 to 5.6","UpgradeStep":"SNAPSHOT"}
```

During the actual upgrade, the `UpgradeStep` is named `UPGRADE`:

```
2020/01/29 08:30:07 [DEBUG] [aws-sdk-go] {"StepStatus":"IN_PROGRESS","UpgradeName":"Upgrade from 5.5 to 5.6","UpgradeStep":"UPGRADE"}
```

We will now only consider upgrade status refresh function `StepStatus` of `SUCCEEDED` only on the `UpgradeStep` value of `UPGRADE`.

Output from acceptance testing:

```
--- PASS: TestAccAWSElasticSearchDomain_basic (2441.12s)
--- PASS: TestAccAWSElasticSearchDomain_ClusterConfig_ZoneAwarenessConfig (5287.37s)
--- PASS: TestAccAWSElasticSearchDomain_CognitoOptionsCreateAndRemove (1881.56s)
--- PASS: TestAccAWSElasticSearchDomain_CognitoOptionsUpdate (2385.75s)
--- PASS: TestAccAWSElasticSearchDomain_complex (1040.02s)
--- PASS: TestAccAWSElasticSearchDomain_duplicate (403.91s)
--- PASS: TestAccAWSElasticSearchDomain_encrypt_at_rest_default_key (760.05s)
--- PASS: TestAccAWSElasticSearchDomain_encrypt_at_rest_specify_key (909.73s)
--- PASS: TestAccAWSElasticSearchDomain_internetToVpcEndpoint (2496.06s)
--- PASS: TestAccAWSElasticSearchDomain_LogPublishingOptions (1598.99s)
--- PASS: TestAccAWSElasticSearchDomain_NodeToNodeEncryption (1015.12s)
--- PASS: TestAccAWSElasticSearchDomain_policy (1832.67s)
--- PASS: TestAccAWSElasticSearchDomain_RequireHTTPS (2784.03s)
--- PASS: TestAccAWSElasticSearchDomain_tags (752.75s)
--- PASS: TestAccAWSElasticSearchDomain_update (1862.46s)
--- PASS: TestAccAWSElasticSearchDomain_update_version (3422.13s)
--- PASS: TestAccAWSElasticSearchDomain_update_volume_type (3397.51s)
--- PASS: TestAccAWSElasticSearchDomain_v23 (1323.39s)
--- PASS: TestAccAWSElasticSearchDomain_vpc (1045.76s)
--- PASS: TestAccAWSElasticSearchDomain_vpc_update (2579.22s)
--- PASS: TestAccAWSElasticSearchDomain_withDedicatedMaster (2644.34s)
```
kjmkznr/terraform-provider-aws,terraform-providers/terraform-provider-aws,terraform-providers/terraform-provider-aws,kjmkznr/terraform-provider-aws,kjmkznr/terraform-provider-aws,terraform-providers/terraform-provider-aws,terraform-providers/terraform-provider-aws,kjmkznr/terraform-provider-aws