Allow venv python interpreter to be overridden

Change-Id: I5a25f8d8beebc63159e740250c7e6e43294e568b

defaults/main.yml

@@ -13,6 +13,9 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+#python venv executable
+rally_venv_python_executable: "{{ openstack_venv_python_executable | default('python2') }}"
+
 # Set the package install state for distribution and pip packages
 # Options are 'present' and 'latest'
 rally_package_state: "latest"

tasks/main.yml

@@ -46,6 +46,7 @@
   import_role:
     name: "python_venv_build"
   vars:
+    venv_python_executable: "{{ rally_venv_python_executable }}"
     venv_build_constraints: "{{ rally_git_constraints }}"
     venv_install_destination_path: "{{ rally_bin | dirname }}"
     venv_install_distro_package_list: "{{ rally_distro_packages }}"