Allow bamboo to start EC2 agents on boot

Currently when bamboo starts there are no EC2 agents configured to start on boot. Agents can be manually (or automatically) shutdown, however once this has occured a new agent will not be spawned in its place.

I would like to propose 3 new options for configuring ec2 agents

1. Min number
2. Max number
3. Max spare

Using these three variables on boot bamboo would automatically start the min number of agents, as builds are triggered, if no agents were available, an additional ec2 agent would be started, up to the max number of configured agents.

Additionally, as build plans are serviced, if there were more than the max spare agents available, agents should by configured to shutdown to reduce the number of compute hours going to waste