amazon-ecs plugin: retry transient ECS task launch failures

[wenduwan]

This patch introduces a retry mechanism that improves the robustness of task launch. A common scenario is elastic network interface provisioning timeout.

Currently the task will be retried once at most, and the total launch time is capped at slaveTimeoutInSeconds.

• [x] Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• [ ] Ensure that the pull request title represents the desired changelog entry: Need guidance on how to add changelog entry
• [x] Please describe what you did: Introduce a ECS task launch retry behavior to overcome transient failures.
• [x] Link to relevant issues in GitHub or Jira: https://github.com/jenkinsci/amazon-ecs-plugin/issues/315
• [N/A] Link to relevant pull requests, esp. upstream and downstream changes
• [x] Ensure you have provided tests - that demonstrates feature works or fixes the issue: Added unit test. Also tested in local jenkins.

[wenduwan]

Hi team I would appreciate your review and critique.