Allow venv python interpreter to be overridden

Change-Id: I68b4c56e7c3ebf2f2520e63b22e86268e8c062e5

defaults/main.yml

@@ -32,6 +32,7 @@ designate_pip_package_state: "latest"
 
 # Set installation method.
 designate_install_method: "source"
+designate_venv_python_executable: "{{ openstack_venv_python_executable | default('python2') }}"
 
 ## The git source/branch
 designate_git_repo: https://git.openstack.org/openstack/designate

tasks/designate_install.yml

@@ -40,6 +40,7 @@
   include_role:
     name: "python_venv_build"
   vars:
+    venv_python_executable: "{{ designate_venv_python_executable }}"
     venv_build_constraints: "{{ designate_git_constraints }}"
     venv_build_distro_package_list: "{{ designate_devel_distro_packages }}"
     venv_install_destination_path: "{{ designate_bin | dirname }}"