Added support for programmatic update of a SlaveTemplate

[drullar]

PR Description

Added functionallity that would allow programmatic update of SlaveTemplates, which can be done by specifying the "old" SlaveTemplate's description. In case a template with the provided description does not exist, an exception is thrown. This feature is useful when someone wants to manage the SlaveTemplates programatically via the Jenkins Script feature.

Testing done

Added an Unit test for the new EC2Cloud method. Attached screenshot of local test execution

Submitter checklist

• 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
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests - that demonstrates feature works or fixes the issue

[drullar]

Note: I am not sure what is your proceedure on updating the ReadMe. If needbe I can make edits and add description for the programmatic addition and update of a SlaveTemplate

[res0nance]

Would you kindly add a section for it in the readme?

[drullar]

Added. Let me know if it needs any corrections