[docs] Add python3-venv package to quickstart.rst

Python virtual environment creation fails on a fresh system if
python3-venv is not installed (Debian/Ubuntu).

Installation instructions used to be in the Quick Start guide, but
were removed in kolla-ansible/+/735894

Change-Id: I867322ee43d509fe46b6afba9216586515fee0d5

doc/source/user/quickstart.rst

@@ -62,6 +62,16 @@ Install dependencies using a virtual environment
 
 If not installing Kolla Ansible in a virtual environment, skip this section.
 
+#. Install the virtual environment dependencies.
+
+   For CentOS or RHEL 8, you don't need to do anything.
+
+   For Debian or Ubuntu, run:
+
+   .. code-block:: console
+
+      sudo apt install python3-venv
+
 #. Create a virtual environment and activate it:
 
    .. code-block:: console