Consider pre-release wheels for installation

When building from git sources or from master, pre-release
wheels may be built. We need to ensure that they are used
if they are present. For stable branches, we are still
protected by the use of the constraints to ensure that we
only get the packages we expect.

Change-Id: I68e6d29cfb6a144d119eabdeb7e23bff73c51555

tasks/python_venv_install.yml

@@ -74,6 +74,7 @@
     state: "{{ venv_pip_package_state }}"
     virtualenv: "{{ venv_install_destination_path }}"
     extra_args: >-
+      --pre
       --log /var/log/python_venv_build.log
       {{ venv_pip_install_args }}
   register: _install_venv_pip_packages