Fix elasticache cluster nuking

[zackproser]

Description

Fixes #341.

• Look up replication groups separately. Previously we were trying to look up all cache nodes via DescribeCacheClusters, which will return nodes that are members of a ReplicationGroup - but won't signify which need to be passed through the ReplicationGroup code path. Instead, we first look up replication groups via their dedicated method DescribeReplicationGroups then issue a call to DescribeCacheClusters, specifying that the call should only return cache nodes that are not members of a replication group.
• When then flatten both cache cluster types together, returning both types from the list method
• When nuking an elasticache cluster - return immediately if the cluster is found to be of type Replication - previously, incorrect fallthrough behavior was causing replication groups to end up in the code path for deleting single cache nodes.

TODOs

Read the Gruntwork contribution guidelines.

• Update the docs.
• Run the relevant tests successfully, including pre-commit checks.
• Ensure any 3rd party code adheres with our license policy or delete this line if its not applicable.
• Include release notes. If this PR is backward incompatible, include a migration guide.

Release Notes (draft)

Added / Removed / Updated [X].

Migration Guide

[yorinasub17]

LGTM! Also confirmed test failures are unrelated.

[zackproser]

Thanks for review! Going to merge this in now.