Probe: restrict requests to certain clusters and/or nodes

Probe.sh broadcasts requests to all reachable nodes in any cluster. The only way to restrict the reach of a probe request is to pass a time-to-live to it, but that's (a) too coarse-grained and (b) doesn't work for TCP.
We should therefore be able to define a list of clusters and a list of nodes to which we want to restrict the request. Both clusters and nodes should be simple regexps.
The filter is sent with the probe request. Nodes which don't pass the filter simply discard the request.

Example:

probe.sh -cluster DrawCluster,cluster-*

This restricts the request to the DrawCluster and any cluster starting with cluster-.

probe.sh -nodes cluster-*

Sends the request only to nodes with a name starting with cluster.