Allow venv python interpreter to be overridden

Change-Id: I413821cbfd10686ddce8d64d156590de5dff2305

defaults/main.yml

@@ -17,6 +17,9 @@
 ## Verbosity Options
 debug: False
 
+#python venv executable
+congress_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

@@ -66,6 +66,7 @@
   import_role:
     name: "python_venv_build"
   vars:
+    venv_python_executable: "{{ congress_venv_python_executable }}"
     venv_build_constraints: "{{ congress_git_constraints }}"
     venv_build_distro_package_list: "{{ congress_devel_distro_packages }}"
     venv_install_destination_path: "{{ congress_bin | dirname }}"