Use --no-cache-dir options for pip install

Use --no-cache-dir option when running the unit tests
and creating the venv to avoid issues with using the
cached package

Change-Id: I0e415705c56ce419c55817d21623b61ca5f65eba
Closes-Bug: #1558776

src/config/common/tests/tools/install_venv_common.py

@@ -108,7 +108,7 @@ def create_virtualenv(self, no_site_packages=True):
     def pip_install(self, find_links, *args):
         find_links_str = ' '.join('--find-links file://'+x for x in find_links)
         cmd_array = ['%stools/with_venv.sh' %(os.environ.get('tools_path', '')),
-                         'python', '.venv/bin/pip', 'install', 
+                         'python', '.venv/bin/pip', 'install', '--no-cache-dir',
                          '--upgrade']
         for link in find_links:
             cmd_array.extend(['--find-links', 'file://'+link])