cbbackupmgr fetches the collection manifests for included/excluded buckets

What's the issue?
Prior to creating the backup plan, cbbackupmgr lists all the source buckets and fetches the collection manifests for each. These are then passed into the bucket mapping creation functions.

Whilst creating the plan, the included/excluded buckets will still be correctly handled but in theory we don't need to pass in the bucket/manifest for buckets that we know we aren't going to be transferring.

What's the fix?
Immediately after fetching the manifests, we read the backup config and warn about missing included/excluded collection strings; we should reorder, and only fetch the collection manifests for buckets that are going to be transferred.