Sync venv scripts from oslo-incubator

Currently install_venv.py and with_venv.sh create virtual
environment in .venv directory and there is no way to specify
the virtual environment path explicitly.

This has been fixed in oslo-incubator.
This patch sync those virtual environment scripts from oslo-incubator.
Also make run_test.sh and run_tempest.sh scripts to use 'VENV' env variable.

Change-Id: I55311cff4dea1f6c5adaa2a6d51eeee2f6cc71ea
Closes-Bug: 1370086
diff --git a/tools/install_venv_common.py b/tools/install_venv_common.py
index 743b59d..e279159 100644
--- a/tools/install_venv_common.py
+++ b/tools/install_venv_common.py
@@ -101,6 +101,7 @@
             print('done.')
         else:
             print("venv already exists...")
+            pass
 
     def pip_install(self, *args):
         self.run_command(['tools/with_venv.sh',
@@ -124,7 +125,7 @@
         parser.add_option('-n', '--no-site-packages',
                           action='store_true',
                           help="Do not inherit packages from global Python "
-                               "install")
+                               "install.")
         return parser.parse_args(argv[1:])[0]