Fix to use . to source script files

Adhering to coding conventions. Refer to ``Code conventions`` at
https://docs.openstack.org/contributor-guide/ for details.
When you have to source a script file, for example, a credentials file
to gain access to user-only or admin-only CLI commands, use . instead of source.
https://docs.openstack.org/contributor-guide/writing-style/code-conventions.html

Change-Id: I97c0cc8b9215b0f580311b1850b13a72f9ced60a

tools/install_venv.py

@@ -35,7 +35,7 @@ def print_help(venv, root):
     To activate the Mistral virtualenv for the extent of your current shell
     session you can run:
 
-    $ source %s/bin/activate
+    $ . %s/bin/activate
 
     Or, if you prefer, you can run commands in the virtualenv on a case by case
     basis by running: