[builder] Add git failure mixin
[builder] Add git failure mixin

* add GitFailureMixin to create GitFailure records when git fulfillments fail
* update git workers to use GitFailureMixin to create GitFailure records
* update minigit service to provide a #subject function that returns the git
  subject for a commit sha

jrafanie/miq_tools_services,ManageIQ/miq_tools_services,imtayadeway/miq_tools_services