Start distributed test with multiple slaves with one command.

Description of feature request

Currently when we need to do distributed test, we have to start Locust master and slaves one by one. Suppose our load test machine has 32 cores, we need to run start command 33 times ! Also, when we adjust our Locust scripts, we have to kill all Locust slaves and start again.

Considering this scenario is so common, we can add one parameter (such as --cpu-cores) to simplify this job.

Expected behavior

With the argument , we can start locust with master and specified number of slaves (default to cpu cores number) at one time.

$ locust -f locustfile.py --cpu-cores 4
[2017-08-26 23:51:47,071] bogon/INFO/locust.main: Starting web monitor at *:8089
[2017-08-26 23:51:47,075] bogon/INFO/locust.main: Starting Locust 0.8a2
[2017-08-26 23:51:47,078] bogon/INFO/locust.main: Starting Locust 0.8a2
[2017-08-26 23:51:47,080] bogon/INFO/locust.main: Starting Locust 0.8a2
[2017-08-26 23:51:47,083] bogon/INFO/locust.main: Starting Locust 0.8a2
[2017-08-26 23:51:47,084] bogon/INFO/locust.runners: Client 'bogon_656e0af8e968a8533d379dd252422ad3' reported as ready. Currently 1 clients ready to swarm.
[2017-08-26 23:51:47,085] bogon/INFO/locust.runners: Client 'bogon_09f73850252ee4ec739ed77d3c4c6dba' reported as ready. Currently 2 clients ready to swarm.
[2017-08-26 23:51:47,084] bogon/INFO/locust.main: Starting Locust 0.8a2
[2017-08-26 23:51:47,085] bogon/INFO/locust.runners: Client 'bogon_869f7ed671b1a9952b56610f01e2006f' reported as ready. Currently 3 clients ready to swarm.
[2017-08-26 23:51:47,085] bogon/INFO/locust.runners: Client 'bogon_80a804cda36b80fac17b57fd2d5e7cdb' reported as ready. Currently 4 clients ready to swarm.

Actual behavior

To achieve the same goal above, we have to start Locust master first.

$ locust -f locustfile.py --master

And then open another terminal shell, start Locust slaves one by one.

$ locust -f locustfile.py --slave &
$ locust -f locustfile.py --slave &
$ locust -f locustfile.py --slave &
$ locust -f locustfile.py --slave &

Environment settings (for bug reports)

N/A

Steps to reproduce (for bug reports)

N/A

[SpencerPinegar]

Yeah, it would be cool to have an example of this.

[MarcSteven]

The issue is still fuzzy, but the command cannot go @debugtalk