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: I1ff56e0775da68e43a691f7f25b3a4ce7930406a

tools/install_venv.py

@@ -39,7 +39,7 @@ def print_help():
  To activate the Neutron virtualenv for the extent of your current shell
  session you can run:
 
- $ source .venv/bin/activate
+ $ . .venv/bin/activate
 
  Or, if you prefer, you can run commands in the virtualenv on a case by case
  basis by running: