Update elasticsearch.py

Correct misspelled variable name.

Throws:

2013-11-08 14:29:12,731 tcollector[27472] WARNING: elasticsearch.py: Traceback (most recent call last):
2013-11-08 14:29:12,731 tcollector[27472] WARNING: elasticsearch.py:   File "/usr/lib/tcollector/collectors/0/elasticsearch.py", line 247, in <module>
2013-11-08 14:29:12,731 tcollector[27472] WARNING: elasticsearch.py:     sys.exit(main(sys.argv))
2013-11-08 14:29:12,731 tcollector[27472] WARNING: elasticsearch.py:   File "/usr/lib/tcollector/collectors/0/elasticsearch.py", line 136, in main
2013-11-08 14:29:12,732 tcollector[27472] WARNING: elasticsearch.py:     if "timestamp" in nststs:
2013-11-08 14:29:12,732 tcollector[27472] WARNING: elasticsearch.py: NameError: global name 'nststs' is not defined