Add parent_id field to projects

Field to point to a parent project. This enables
the possibility to use hierarchical projects.

Co-Authored-By: Rodrigo Duarte <rodrigods@lsd.ufcg.edu.br>
Co-Authored-By: Raildo Mascena <raildo@lsd.ufcg.edu.br>
Co-Authored-By: Telles Mota Vidal Nobrega <tellesmvn@lsd.ufcg.edu.br>

Implements: blueprint hierarchical-multitenancy
Change-Id: I2b56a362df40b664e3a802d09ca027277678b300