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 implements a new variable 'venv_build_global_constraints' which
is a list of constraints to be applied globally for all venvs. This list
will be used to produce a file in the venv suffixed with
'-global-constraints.txt' and will be used on the pip command line when
building the wheels and when installing packages.
We also ensure that all constraints are used when both building and
installing pip, setuptools and wheel into the venv.
Change-Id: I9ae3ef19c863b9237a51d2fcd6f4ebce1a9ebad7
cce10ac38e