In the previous repo build process, we had global constraints which
override upper constraints and anything set in the roles. This was
essential for two purposes:
1. To enable us to pin things that were not in upper constraints. eg: pip,
setuptools, wheel
2. To enable us to pin things which were in upper constraints, but broken.
This would usually be a temporary measure until upper constraints was
fixed.
This patch extracts the global pins from global-requirement-pins.txt
into a list and sets 'venv_build_global_constraints' to the resulting
list so that it is applied to all venvs built.
In order to reliably find this file without using a hard-coded path, we
implement a change to the wrapper script to set the path in it in a
similar manner to that implemented for the inventory path.
Depends-On: I9ae3ef19c863b9237a51d2fcd6f4ebce1a9ebad7
Change-Id: I138fe1c8ea80fe71244ab0dc6497cfc6d7bdf953