Fix unnecessary decrement of maxTotalUses and add clarity to logs around terminations triggered by plugin

[pdk27]

Issue

#363

Changes

• add and track EC2AgentTerminationReason for cases when plugin triggers termination
• [fix] remove unnecessary decrement which could lead to a positive maxTotalUses eventually decrement to -1.
  '-1' has special meaning, means unlimited uses which is the opposite of the maxTotalUses.
• [fix] remove misleading log to prevent logs like Agent xxx is still in use by more than one (1) executors.
• [fix] remove redundant computer.setAcceptingTasks(false)
• rename
  • EC2TerminationCause to EC2ExecutorInterruptionCause for clarity
  • rename force to overrideOtherSettings in scheduleToTerminate for clarity

Testing done

Tested all changes with a snapshot version of the plugin.

Extra logs look like

[INFO  ][com.amazon.jenkins.ec2fleet.EC2FleetCloud info] EC2FleetCloud1 [spot-workers] Scheduling instance 'i-xxx' for termination on cloud com.amazon.jenkins.ec2fleet.EC2FleetCloud@xxx because of reason: MaxTotalUses exhausted for agent  

[FINE  ][com.amazon.jenkins.ec2fleet.EC2FleetCloud fine] EC2FleetCloud1 [spot-workers] InstanceIdsToTerminate: {i-xxx=MaxTotalUses exhausted for agent}

[FINE  ][com.amazon.jenkins.ec2fleet.EC2FleetCloud fine] SpotFleetCloud1 [spot-workers-2] InstanceIdsToTerminate: {i-yyy=Agent idle for too long}