Ensure maxAgents is respected when requesting additional agents.

[tomcart90]

This change aims to fix the issues reported on https://github.com/jenkinsci/amazon-ecs-plugin/issues/269. Essentially when requesting additional agents the excessWorkload value is passed regardless of the maxAgents limit. When triggering parallel jobs excessWorkload may in fact be higher than the number of available agents when maxAgents is set.

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[uorbe001]

Our ci is constantly triggering hundreds or thousands of excess agents (which is causing total collapse), while being limited to 10 agents and it seems to be for the same reason fixed here. Is there anything that can be done to help this get merged?

[tomcart90]

Thanks for shepherding this through @Stericson!