Allow venv python interpreter to be overridden

Change-Id: I37300557a4318e7468298b5575e7dec02bae7eff

defaults/main.yml

@@ -14,6 +14,9 @@
 ## Verbosity Options
 debug: False
 
+#python venv executable
+sahara_venv_python_executable: "{{ openstack_venv_python_executable | default('python2') }}"
+
 # Set the host which will execute the shade modules
 # for the service setup. The host must already have
 # clouds.yaml properly configured.

tasks/main.yml

@@ -65,6 +65,7 @@
   import_role:
     name: "python_venv_build"
   vars:
+    venv_python_executable: "{{ sahara_venv_python_executable }}"
     venv_build_constraints: "{{ sahara_git_constraints }}"
     venv_install_destination_path: "{{ sahara_bin | dirname }}"
     venv_install_distro_package_list: "{{ sahara_distro_packages }}"