Add pipeline_slug and disabled_reason in DOWNSTREAM_PROJECTS (#389)

* Add pipeline_slug and downstream_enabled in DOWNSTREAM_PROJECTS

1. Add pipeline_slug for every project configuration, this allows us to
   get the last green commit later.
2. Add downstream_enabled attribute, easier to disable a project in
   downstream.
3. Only update last green commit for project existing in
   DOWNSTREAM_PROJECTS.

Change-Id: Ia531d8d19fc4096bbbbd6ec2c370291d016baefc

* Use disabled_reason instead of downstream_disabled

Change-Id: I6afd73a0e9a11238fceff91b61e38bfb1caaa332