Fix: `load_cluster()` does not find existing cluster.

`load_cluster()` uses "artist" for looking up existing clusters, but
Cluster does not have a meta-data item "artist".

This bug does not show up when clustering all files at once (which
will be the case most times), as in this case a single cluster will be
calculated and only looked up once. But when clustering files in two
steps, a second cluster will be generated as the first one will not be
found.

Signed-off-by: brainz34 <brainz34@musicbrainz.org>