Cap Pip<8 due to pip bug

Due to pypa/pip#3384 in pip v8, this change
puts a cap on pip, in a similar manner to
https://review.openstack.org/#/c/269954/

Change-Id: I20b8b597e8806a7ee21d947eccc98e4ae7e00c3b
Closes-Bug: 1536290

tools/install_venv_common.py

@@ -113,7 +113,7 @@ def install_dependencies(self):
 
         # First things first, make sure our venv has the latest pip and
         # setuptools and pbr
-        self.pip_install('pip>=1.4')
+        self.pip_install('pip<8,>=1.4')
         self.pip_install('setuptools')
         self.pip_install('pbr')