Fix zuul.(yaml|d) loading order and add tests

This change fixes the configloader to load file in a more logical order:
zuul.yaml, zuul.d, .zuul.yaml then .zuul.d.

This change also adds negative test for config path conflicts.

Change-Id: Ifbbfb4389b7aa0a641b95b5947abc8a56e362ee3
9 files changed