[#326] Set number of executors on ECS Agent

[k3np]

The number of executors on ECS Agent is currently hardcoded to 1 even though the number of executors is configurable is the ECS Cloud settings. This fix sets the number of executors from the ECS Cloud settings.

Fixes #326

Testing done

• Added tests for both default (1) executor and multiple executors.
• Plugin build and tested on our own Jenkins instances. Multiple jobs can be executed in parallel on single ECS Agent.
• Confirmed settings in Script Console:

  
  // Print NumExecutors in ECS Cloud settings
  println "NumExecutors: " + com.cloudbees.jenkins.plugins.amazonecs.ECSCloud.getByName("ecs").getNumExecutors()
  // Result: NumExecutors: 4

// Print NumExecutors in ECS Agent jenkins.model.Jenkins.instance.nodes.each { node -> println "NumExecutors: " + node.getNumExecutors() } // NumExecutors: 4 // Result: [com.cloudbees.jenkins.plugins.amazonecs.ECSSlave[ecs-test-pjt74]]

```[tasklist]
### Submitter checklist
- [x] Make sure you are opening from a **topic/feature/bugfix branch** (right side) and not your main branch!
- [x] Ensure that the pull request title represents the desired changelog entry
- [x] Please describe what you did
- [x] Link to relevant issues in GitHub or Jira
- [x] Link to relevant pull requests, esp. upstream and downstream changes
- [x] Ensure you have provided tests - that demonstrates feature works or fixes the issue

[nbr23]

This is a pretty critical fix, thanks so much! Any chance of getting it merged and released soon?

Thanks!

[vishal-elimu]

@kp-autorola Thanks for the proposed fix ! It really affects the use of ecs managed jenkins-agents. One request - Is it possible for you to add reviewers to this pull request ?

[k3np]

@vishal-elimu I don't have access to assign reviewers.

[vishal-elimu]

Ok @kp-autorola - Thanks for the update !

[vishal-elimu]

Hi - @pgarbe @jequals5 @carlosrodf is it possible for someone to add reviewer(s) to this pull request ?

[vishal-elimu]

Anyone know How to "Request a pull request review" ?