In order to do a developer mode that allows installation of packages
from local file, which take precedence over the developer_mode
constraints, we need to allow the order of the constraints to be changed.
This patch adds a "pip_install_developer_constraints" var which is used
to set the developer mode constraints. By default this will leave the
same behaviour but will allow additional constraints to be added, or the
developermode constraints file to be overriden altogether.
Change-Id: Iad94ffc7c3dd96a585b94d58ed8997f13d765f0d
5e6440cb0b