fix issue419: work with collection indices instead of node ids.

This reduces network message size.

testing/test_remote.py

@@ -154,7 +154,7 @@ class TestSlaveInteractor:
         assert ev.kwargs['topdir'] == slave.testdir.tmpdir
         ids = ev.kwargs['ids']
         assert len(ids) == 1
-        slave.sendcommand("runtests", ids=ids)
+        slave.sendcommand("runtests", indices=range(len(ids)))
         slave.sendcommand("shutdown")
         ev = slave.popevent("logstart")
         assert ev.kwargs["nodeid"].endswith("test_func")