Update pip in DLRN venv

Newer versions of setuptools require an updated pip to be installed.
Running the --delorean-setup locally is actually failing for me with
(not sure what the difference is compared to CI, but probably is due to
using the infra created CentOS image which has updated pip/virtualenv
already):

ImportError: No module named six.moves

This is an expected problem with newer versions of setuptools per
pypa/setuptools#942 (comment)
pypa/setuptools#937

We're already using the latest packaged versions of virtualenv and pip
on CentOS so the fix is to first update pip in the newly created
virtualenv.

Change-Id: Ibc0e266d48dec587c8221f8392f060b58496def4

scripts/tripleo.sh

@@ -402,6 +402,7 @@ function delorean_setup {
  # NOTE(pabelanger): We need to update setuptools to the latest version for
  # CentOS 7. Also, pytz is not declared as a dependency so we need to
  # manually add it. Lastly, use pip install . to use wheel AFS pypi mirrors.
+ ./venv/bin/pip install -U pip
  ./venv/bin/pip install -U setuptools
  ./venv/bin/pip install pytz
  ./venv/bin/pip install .