Doesn't collect RDS db-clusters

Please mention the following:
- What command was run? `python cloudmapper.py collect --account myacct`
- Are you working out of a pipenv environment, Docker, or something else? `pipenv`

I have an [Aurora Serverless](https://docs.aws.amazon.com/AmazonRDS/latest/AuroraUserGuide/aurora-serverless.html) database. This does not show up as an instance but only as a cluster:
```
(venv) cloudmapper$ aws rds describe-db-instances
{
"DBInstances": []
}
(venv) cloudmapper$ aws rds describe-db-clusters
{
"DBClusters": [
{
"AllocatedStorage": 1,
"AvailabilityZones": [
"us-east-1e",
"us-east-1b",
"us-east-1c"
],
"BackupRetentionPeriod": 1,
"DatabaseName": "xxx",
"DBClusterIdentifier": "xxx",
"DBClusterParameterGroup": "default.aurora5.6",
"DBSubnetGroup": "default",
"Status": "available",
...
"VpcSecurityGroups": [
{
"VpcSecurityGroupId": "sg-xxxxxxxxx",
"Status": "active"
}
],
...
```

It appears that `collect_commands.yaml` does not include db-instances:
```
(venv) cloudmapper$ grep -nr describe-db-cluster .|grep -v venv
(venv) cloudmapper$ grep -nr describe-db-instance .|grep -v venv
./collect_commands.yaml:121: Request: describe-db-instances
./collect_commands.yaml:133: Value: rds-describe-db-instances.json|.DBInstances[]?|.DBInstanceArn
./shared/audit.py:625: json_blob = query_aws(region.account, "rds-describe-db-instances", region)
./commands/prepare.py:129: instances = query_aws(region.account, "rds-describe-db-instances", region.region)
./stats_config.yaml:15: source: rds-describe-db-instances
(venv) cloudmapper$
```
I expect you'd want to know about clusters even for regular RDS instances that are clustered...