Convert url for getting tasks to relative

[5imun]

I was using Locust UI with kubernetes and after setting up ingress rule to expose the service on custom path I have noticed periodic 404 errors in the browser console occurring each second.

This happens because when using urls with absolute path that starts with / custom base path used for exposing the service in ingress rules gets lost so instead of 200 http://domain.com/locust/tasks we get 404 http://domain.com/tasks.
This change will make it relative so that base path /locust in example before will be preserved.

This is also related to issues #149, #2030 and PR 692 that solved this problem for code before 30 May 2018.
This will also solve issue #2071 reported by @benispeti

[cyberw]

thx