null (Service: AmazonRDS; Status Code: 500; Error Code: InternalFailure when launching documentdb

**Describe the bug**

AWS Cloudformation Create/Update Stack fails with 500 error when creating a document DB cluster with null (Service: AmazonRDS; Status Code: 500; Error Code: InternalFailure
CreateDBCluster' api call isn't being issued when we check Cloudtrail
**To reproduce**
In AzureDevops
Create a Pipeline
Under tasks, Add task AWS Cloudformation Create/Update Stack
Use the template provided either as a LocalFile in Template Source, or source it from S3Bucket.
Inline Parameters, reference the provided parameters from the Template
[
{ "ParameterKey":"VpcId", "ParameterValue":"$(VpcId)"},
{ "ParameterKey":"DocDBSubnets", "ParameterValue":"$(DocDBSubnets)"},
{ "ParameterKey":"DocDBInstanceSize", "ParameterValue":"$(DocDBInstanceSize)"},
{ "ParameterKey":"DocDBUsername", "ParameterValue":"$(DocDBUsername)"},
{ "ParameterKey":"DocDBPassword", "ParameterValue":"$(DocDBPassword)"}
]
Under Capabilities specify
CAPABILITIES_IAM and CAPABILITY_NAMED_IAM
Create Release
Pipeline Error: Stack creation request failed with error: 'Stack my-stack failed to reach completion status, error: 'Resource is not in the state stackCreateComplete'' Error: Stackmy-stack failed to reach completion status, error: 'Resource is not in the state stackCreateComplete

**Expected behavior**
Cloudformation stack is created with DocumentDB

**Screenshots**


Here is the template
[doc.txt](https://github.com/aws/aws-vsts-tools/files/4037619/doc.txt)

**Your Environment**
- AWSTools for Microsoft Visual Studio Team Services 1.5.0 running on Azure Devops

**Additional context**
The same template used to work without issues using the VSTS Tools version 14.0.
This template works locally via cli, powershell
Only fails on version 15.0

Start with the AWS CloudFormation Create/Update Stack task in Azure DevOps and reproduce the failure using the attached doc.txt template and parameters. Check CloudTrail for the missing CreateDBCluster call and compare version 15.0 with 14.0; done means the DocumentDB stack reaches stackCreateComplete.

Written by the indexing model from the issue text.