[QA] Browser test are now throttled

[Antoine Musso]

Hello,

The browser tests running on Jenkins are often failing due to some
mysterious errors (timeout, network access, machine overloaded, race
condition or whatever).

Earlier today I have deployed a change that restrict one build per
Jenkins slave.  That might help at the expense of slowing down the time
it takes for all jobs to complete.

We will see.

-- 
Antoine "hashar" Musso