Allow venv python interpreter to be overridden

Change-Id: I438a3b5757ba17e7e54aa9b5e03334784e5ef651

defaults/main.yml

@@ -14,6 +14,7 @@
 
 # Set installation method
 placement_install_method: "source"
+placement_venv_python_executable: "{{ openstack_venv_python_executable | default('python2') }}"
 
 # Set the package install state for distribution and pip packages
 # Options are 'present' and 'latest'

tasks/placement_install.yml

@@ -40,6 +40,7 @@
   import_role:
     name: "python_venv_build"
   vars:
+    venv_python_executable: "{{ placement_venv_python_executable }}"
     venv_build_constraints: "{{ placement_git_constraints }}"
     venv_build_distro_package_list: "{{ placement_devel_distro_packages }}"
     venv_install_destination_path: "{{ placement_bin | dirname }}"