Limits the number of concurrent vm creations to make sure vms are create...

We are using Vagrant-aws to create anywhere between 8-60 machines in parallel. A large number of instances begin to fail when there are more than 10-11 simultaneous calls. In order to allow this to happen, introduced class variable that keeps track of instances and allow only 10 creations at a time. We also saw instance timeout failures so increased the wait_for timeout. This setup is currently working for upto 60 parallel VMs. Let me know your thoughts.