MT#33006 repoapi: support mantis
* tracker app, move everything related to WF or Mantis there
* move settings to tracker:
- REPOAPI_TRACKER -> TRACKER_PROVIDER
- MANTIS_* -> TRACKER_MANTIS_*
- WORKFRONT_* -> TRACKER_WORKFRONT_*
Change-Id: If3eba4e86ec90d6cb4259104d0f474e936f70d43
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