Install a cluster to suppress warning about missing directory.

[linus-sherrill]

The following warning appears all the time because the directory is not there.

Warning - Path not found loading clusters: /usr/local/share/sprokit/pipelines/clusters

The warning can not be removed because there may really be a misconfigured cluster include path. Is this really the best place to install clusters?

This warning has tripped up two external customers who, foolishly, thought that a warning was serious.