Ensure python pip is installed before zuul

Previously, installing zuul resulted in the erros:

  /usr/bin/python: No module named pip

This commit ensures that python-pip is always installed
before zuul is installed so that it can install
correctly.

Change-Id: I6d73a67e1fbb9eb077f39f99c45ceb4a00bc652b
Merge "Ensure python pip is installed before zuul"