Fix/on start run one time

[gunesmes]

fix for: #529

Printing ON_START running inside on_start before fix:
`
~/P/m/p/responsive_gecis (master↑) locust -f all_links.py --host=http://test.com --clients=5 --hatch-rate=1 --num-request=25 --no-web --only-summary
[2017-02-06 18:15:16,959] Mesuts-MacBook.local/INFO/locust.main: Starting Locust 0.7.5
[2017-02-06 18:15:16,959] Mesuts-MacBook.local/INFO/locust.runners: Hatching and swarming 5 clients at the rate 1 clients/s...
[2017-02-06 18:15:16,959] Mesuts-MacBook.local/INFO/stdout: ON_START running
[2017-02-06 18:15:16,960] Mesuts-MacBook.local/INFO/stdout:
[2017-02-06 18:15:17,959] Mesuts-MacBook.local/INFO/stdout: ON_START running
[2017-02-06 18:15:17,960] Mesuts-MacBook.local/INFO/stdout:
[2017-02-06 18:15:18,962] Mesuts-MacBook.local/INFO/stdout: ON_START running
[2017-02-06 18:15:18,962] Mesuts-MacBook.local/INFO/stdout:
[2017-02-06 18:15:19,963] Mesuts-MacBook.local/INFO/stdout: ON_START running
[2017-02-06 18:15:19,964] Mesuts-MacBook.local/INFO/stdout:
[2017-02-06 18:15:20,966] Mesuts-MacBook.local/INFO/stdout: ON_START running
[2017-02-06 18:15:20,966] Mesuts-MacBook.local/INFO/stdout:
[2017-02-06 18:15:21,968] Mesuts-MacBook.local/INFO/locust.runners: All locusts hatched: User: 5
[2017-02-06 18:15:21,969] Mesuts-MacBook.local/INFO/locust.runners: Resetting stats

[2017-02-06 18:15:31,089] Mesuts-MacBook.local/INFO/locust.runners: All locusts dead

[2017-02-06 18:15:31,089] Mesuts-MacBook.local/INFO/locust.main: Shutting down (exit code 0), bye
`

after fix:
`
python /Users/mesutgunes/Projects/locust/locust.py -f all_links.py --host=http://test.com --clients=5 --hatch-rate=1 --num-request=25 --no-web --only-summary
[2017-02-06 18:13:51,758] Mesuts-MacBook.local/INFO/locust.main: Starting Locust 0.8a2
[2017-02-06 18:13:51,758] Mesuts-MacBook.local/INFO/locust.runners: Hatching and swarming 5 clients at the rate 1 clients/s...
[2017-02-06 18:13:51,759] Mesuts-MacBook.local/INFO/stdout: ON_START running
[2017-02-06 18:13:51,759] Mesuts-MacBook.local/INFO/stdout:
[2017-02-06 18:13:56,768] Mesuts-MacBook.local/INFO/locust.runners: All locusts hatched: User: 5
[2017-02-06 18:13:56,768] Mesuts-MacBook.local/INFO/locust.runners: Resetting stats

[2017-02-06 18:14:07,871] Mesuts-MacBook.local/INFO/locust.runners: All locusts dead

[2017-02-06 18:14:07,871] Mesuts-MacBook.local/INFO/locust.main: Shutting down (exit code 0), bye.
Name # reqs # fails Avg Min Max | Median req/s

`

[cgoldberg]

this isn't a fix... it just changes behavior of on_start to something completely different. on_start is supposed to run for each locust before tasks are scheduled. I think you have misunderstood the purpose of on_start.

rejecting this and will not merge.