Update controller logic to include single node selection with prefix

[kshave]

Description

Please provide a meaningful description of what this change will do, or is for. Bonus points for including links to
related issues, other PRs, or technical references.

Update the controller to be able to schedule a test to a single worker node via a nodename prefix. This functionality is useful for users who wish to schedule a test to a single node with a supplied prefix instead of all nodes with a given prefix.

Current node selector functionality:

• node: * - Run on all nodes
• node: worker* - Run on all nodes with the prefix 'worker'
• node: $ - Run on a single (random) node
• node: worker$ - error: no valid agents found to run the synthetic test on, retry after 1 minute ! <--- This PR will fix this

Type of Change

• Bug Fix
• New Feature
• Breaking Change
• Refactor
• Documentation
• Other (please describe)

Checklist

• I have read the contributing guidelines
• Existing issues have been referenced (where applicable)
• I have verified this change is not present in other open pull requests
• Functionality is documented
• All code style checks pass
• New code contribution is covered by automated tests
• All new and existing tests pass

[subbaksh]

This looks good :)
Thank you! 🙏