Add TryToDeleteWorkspace method and replacee existing workspace delete m...

Refactored the workspace.Delete() action into a TryToDeleteWorkspace method. Includes alternate Retry.Do() values of 5 second delay between tries to help with busy TFS servers. Refined this pull request to minimise code changes as per @spraints and @pmiossec suggestions. Should help improve cloning reliability identified in #570, #461.