Allow venv python interpreter to be overridden

Also allow a default var to be specified per distro, maintaining
the current functionality for tripleo without requiring an override.

Change-Id: I99e0082a748d62ed129a860c6f16c523dec8d50e

defaults/main.yml

@@ -23,6 +23,7 @@ stestr_executable: "{{ _stestr_executable | default('stestr') }}"
 # on choosing source it will install from git or venv
 # on choosing distro it will install based on distribution
 tempest_install_method: "source"
+tempest_venv_python_executable: "{{ openstack_venv_python_executable | default(_tempest_venv_python_executable) | default('python2') }}"
 
 # Set the package install state for distribution and pip packages
 # # Options are 'present' and 'latest'

tasks/tempest_install_source.yml

@@ -20,6 +20,7 @@
   import_role:
     name: "python_venv_build"
   vars:
+    venv_python_executable: "{{ tempest_venv_python_executable }}"
     venv_build_constraints: "{{ tempest_git_constraints }}"
     venv_install_destination_path: "{{ tempest_venv_bin | dirname }}"
     venv_pip_install_args: "{{ tempest_pip_install_args }}"

vars/redhat-8.yml

@@ -25,7 +25,7 @@ _stestr_executable: stestr-3
 # virtualenv. By default venv_python_executable var sets to python2, changing it
 # in the role itself, breaks other roles. So for distros using python3 making the
 # change here fixes the issue.
-venv_python_executable: "python3"
+_tempest_venv_python_executable: "python3"
 
 tempest_service_distro_packages:
   - openstack-tempest