Feature/rds docdb fix

[tarunmenon95]

Fixed bug where calling any RDS cluster function would include docdb resource's if they were present. As such docdb snapshots would be created and removed if any RDS cluster actions were invoked.

Example of issue:

1. For this example we have a stack with both a docdb and rds_cluster resource. No snapshots have been created yet for either.
   

2. We call shelvery create_backups rds_cluster

3. We can observe in the logs immediately that shelvery has picked up the docdb_cluster as a resource to backup alongside our actual rds cluster.
   

4. Shelvery creates a snapshot of the doc db cluster aswell as a snapshot for the actual RDS cluster.
   
   

Example of fix:

The solution was to remove any clusters from the list of clusters returned which had Engine='docdb'

1. With the same resources as before with no snapshots existing.
2. We call shelvery create_backups rds_cluster
3. This time we observe that only 1 cluster resource is picked up which is our actual RDS cluster as expected. DocDB is omitted as expected.
   
4. Observe no DocDB snapshot is created
   
5. RDS Cluster snapshot is created as expected