Docker-machine locks main queue

I were investigating the scalability of our docker-machine based method and I did find that since we are using docker-machine ls -qon large installations it takes about 3-4s to finish. Since this is executed in main queue it takes significant amount of time and slows down the maximum number of builds that can be processes by runner to about 500-600 for one hour.

Maybe there's a better way to improve that? Maybe we could move execution of Acquire method of docker+machine to method where we are requesting a build? Maybe we could execute docker-machine ls -q only once instead of executing it every time?

@tmaczukin What do you think?