AWS Credentials - Use Variable in deployment group fails

**Describe the bug**
I have a deployment with multiple stages which are deployed to individual AWS organizations. Thus, I need to specify specific connections for each stage. So I set the AWS credential to `$(AWSAccount)`. This works for normal deployments. However, it fails when used in a deployment-group deployment.

It fails with this log:
```
2019-11-06T14:51:00.6561140Z Configuring credentials for task
2019-11-06T14:51:00.6561308Z ##[debug]awsCredentials=AWS - Staging
2019-11-06T14:51:00.6561424Z ##[debug]task result: Failed
2019-11-06T14:51:00.6592575Z ##[error]Endpoint auth data not present: AWS - Staging
2019-11-06T14:51:00.6592978Z ##[debug]Processed: ##vso[task.issue type=error;]Endpoint auth data not present: AWS - Staging
2019-11-06T14:51:00.6616183Z ##[debug]Processed: ##vso[task.complete result=Failed;]Endpoint auth data not present: AWS - Staging
2019-11-06T14:51:00.6618671Z AWS - Staging exists false
2019-11-06T14:51:00.6619389Z ##[debug]AWS - Staging exists false
2019-11-06T14:51:00.6619622Z ...configuring AWS credentials from service endpoint 'AWS - Staging'
```

If I enter the actual endpoint directly, the log looks like this:
```
019-11-06T14:28:09.1626091Z Configuring credentials for task
2019-11-06T14:28:09.1626889Z 1f20638e-2d31-4a06-9230-f6a63f123089 exists true
2019-11-06T14:28:09.1627728Z ...configuring AWS credentials from service endpoint '1f20638e-2d31-4a06-9230-f6a63f123089'
2019-11-06T14:28:09.1627972Z ...endpoint defines standard access/secret key credentials
```

But this only happens on a Deployment Group job. It seems that the connection name is not being properly translated to its Id when run on a Deployment Group Job.

**To reproduce**
Use an AWS task in a deployment group job. Specify the AWS credential via a variable like `$(AWSAccount)` and set the variable value to the name of an AWS credential.

**Expected behavior**
The task executes successfully.

**Your Environment**
Azure Devops Server 2019, on-prem
AWS Tools 1.50

Start by reproducing an AWS task in an Azure DevOps deployment-group job with the credential set to a variable, then compare its credential-resolution logs with a direct endpoint configuration. Trace the deployment-group credential path and verify that the variable's endpoint name resolves to the service endpoint ID and the task completes successfully.

Written by the indexing model from the issue text.