Expose status_url parameter in zuul_prod/dev class signature

The status_url is not exposed in the signature, and it's
used a hardcoded value in the nested ::zuul class declaration.
This allows user to define their own value.

Change-Id: Ibf0aafaa4c7d86a5d3ba71018b93427e6a8c744e
Closes-Bug: 1387994
Merge "Expose status_url parameter in zuul_prod/dev class signature"