The `CSIPlugin.List` RPC was intended to accept a prefix to filter the
list of plugins being listed. This was being accidentally being done
in the state store instead, which contributed to incorrect filtering
behavior for plugins in the `volume plugin status` command.
Move the prefix matching into the RPC so that it calls the
prefix-matching method in the state store if we're looking for a
prefix.
Update the `plugin status command` to accept a prefix for the plugin
ID argument so that it matches the expected behavior of other commands.
Tim Gross