We pull the pip constraints from the requirements repo so need to clone
that repo prior to using the constraints. In fixup_stuff.sh devstack
attempts to install packages like prettytable using the constraints. It
is also possible to need constraints before fixup_stuff.sh if tracking
depends. To deal with this clone requirements repo before any possible
use of constraints in pip_install.
Change-Id: I42e981c8c5ce1b8a57b9f6cce213065c72d6af11
57aafb5a9a