Add the ability to limit maximum active Elastic Agents using a single configuration

Problem Definition

If mounting an EBS volume in an Elastic Image configuration, there is not a way to tell Bamboo to only ever have a single instance active using that configuration.

When an instance is active with a particular configuration and another build is triggered, which uses that config, one of two things happens:

1. Build is put in queue to be used on that agent (OK)
2. Bamboo attempts to spin up a new EC2 instance using the configuration. Then errors out because the EBS volume is already in use by another EC2 instance.

Suggested Solution

Add an option in the image configuration to limit the number of concurrent copies of a single image configuration.

Workaround

1. If the data on the EBS volume is Static, create an EBS snapshot to mount to the Elastic Agent Instance, rather than mounting a defined volume. This will allow Bamboo to create a new volume based on the snapshot for each agent instance.
   Configuring elastic instances to use the EBS
2. If the data on the EBS volume frequently changes, you'll need to create a separate EBS volume for each build, and create individual Elastic Image configurations and dedicate each image config to an individual build with its own unique EBS volume.