Fix the race condition in the image cap calculation

Use of a static synchronized hashmap `provisioningAmis` to keep track of the
number of containers per images being provisioned.
This code mainly comes from the jenkins ec2-plugin so all the credit goes to
its authors.
GH-28 Add a container cap by machine
Merge pull request #34 from dreewoo/instance_cap

Add a per machine instance cap and fix instance cap limit calculation