Run time input for web-ui

[ajt89]

Describe the solution you'd like

Extend the existing "/swarm" endpoint in web.py to have another field for runtime and also add an option in the web-ui itself. Essentially try to do --autostart in the web-ui

Describe alternatives you've considered

• An external tool such as jenkins or another python app to call the /swarm, sleep for some amount of time, and call /stop
• Re-deploy the locust cluster (master, and workers) with --autostart every time I want a timed test to run

[cyberw]

Good idea, PR:s welcome.

[cyberw]

Fixed