Simplify schedulers.
Refactor schedulers, remove schedulers, use generic scheduler.
Add a rate limiter, use it to rate limit docker pulls.
Add ctx to registry interfaces
Merge pull request #1449 from brendandburns/scheduler

Refactor scheduler, use generic scheduler everywhere, delete old code.
Add ctx to registry test cases
Merge pull request #1457 from brendandburns/pull

Add a rate limiter, use it to rate limit docker pulls.
Merge pull request #1463 from derekwaynecarr/introduce_context_obj

Add ctx object to registry interface
Fix the vagrant build.
Merge pull request #1466 from brendandburns/vagrant

Fix the vagrant build.