Restore support for per-job repository URL, username + password.

Modify repo, username + password defaulting so it falls back at runtime, not at job-creation time.
Changed workspaceName templating to DRY up code and make it work with any Jenkins variables, not just NODE_NAME and JOB_NAME.
Ensured there's help for all these config values.
Unit tests for code changes.
Merge pull request #11 from RoystonS/master

Restore support for job-specific username, password, repository details.