couchbase-cli can't update metadata purge interval at bucket level

Summary: On ephemeral buckets, Metadata Purge Interval can be specified via the UI. When using couchbase-cli, however, there is no option to specify a bucket when using setting-compaction.

Repro:

1. Install Couchbase Server Community 5.1.1 (this may affect other versions too)
2. Create an ephemeral bucket called 'foo'
3. Under 'Advanced bucket settings' set the Metadata Purge Interval to 0.0777 (for instance)
4. From the command line, run `couchbase-cli setting-compaction -c 127.0.0.1 -u Administrator -p password --metadata-purge-interval 0.0625

Expected:

The bucket 'foo' to have its Metadata Purge Interval changed.

Actual:

The bucket 'foo's Metadata Purge Interval is not changed.

Notes:

I can't tell if this is missing behavior (i.e. there's no -b bucket option for setting-compaction) or if this is a bug.