Nodes must exist in node_shard_map!

Fixes:

    Unhandled Python exception: KeyError(<__main__.ESNode object at 0x2964fd0>,)

      File "/usr/lib/pymodules/python2.6/nagioscheck.py", line 246, in run
        self.check(opts, args)
      File "/usr/local/sbin/check_elasticsearch", line 466, in check
        my_shards = set(node_shard_map[esid_node_map[myid]])