Purge logstash data older than a configurable number of days; use isvcs:v17

DEMO:

```
$ zendev serviced -dx
...
I0917 13:56:59.433386 02985 es.go:144] Purging logstash entries older than 14 days
/bin/bash: warning: setlocale: LC_ALL: cannot change locale (en_US.UTF-8)
2014-09-17 13:57:00,027 INFO      Job starting...
2014-09-17 13:57:00,031 INFO      Beginning DELETE operations...
2014-09-17 13:57:00,036 INFO      logstash-2014.09.17 is within the threshold period (14 days).
2014-09-17 13:57:00,036 INFO      DELETE index operations completed.
2014-09-17 13:57:00,036 INFO      Done in 0:00:00.018674.

$ zendev serviced -dx -- --logstash-max-days=2
...
I0917 13:58:46.368207 05401 es.go:144] Purging logstash entries older than 2 days
/bin/bash: warning: setlocale: LC_ALL: cannot change locale (en_US.UTF-8)
2014-09-17 13:58:47,000 INFO      Job starting...
2014-09-17 13:58:47,004 INFO      Beginning DELETE operations...
2014-09-17 13:58:47,010 INFO      logstash-2014.09.17 is within the threshold period (2 days).
2014-09-17 13:58:47,010 INFO      DELETE index operations completed.
2014-09-17 13:58:47,011 INFO      Done in 0:00:00.022001.

$ SERVICED_LOGSTASH_MAX_DAYS=4 zendev serviced -dx
...
I0917 14:00:42.464749 07846 es.go:144] Purging logstash entries older than 4 days
/bin/bash: warning: setlocale: LC_ALL: cannot change locale (en_US.UTF-8)
2014-09-17 14:00:42,912 INFO      Job starting...
2014-09-17 14:00:42,916 INFO      Beginning DELETE operations...
2014-09-17 14:00:42,922 INFO      logstash-2014.09.17 is within the threshold period (4 days).
2014-09-17 14:00:42,922 INFO      DELETE index operations completed.
2014-09-17 14:00:42,923 INFO      Done in 0:00:00.021805.
```
Done.