Build wheels only for source isntalls

Ubuntu distro installation installs tempest plugins from source.
For this scenario we're  ensuring that wheels won't be built as
repo container that should handle wheels build is simply absent in this
scenario.

For the rest usecases we're sticking with our new default to build
wheels.

Change-Id: Id643946edf4b4e21f8420d048ce8293fc40a61fc

tasks/tempest_install_source.yml

@@ -40,7 +40,7 @@
     venv_install_destination_path: "{{ tempest_venv_bin | dirname }}"
     venv_build_distro_package_list: "{{ tempest_devel_distro_packages }}"
     venv_pip_install_args: "{{ tempest_pip_install_args }}"
-    venv_wheel_build_enable: "{{ venv_build_host != inventory_hostname }}"
+    venv_wheel_build_enable: "{{ tempest_install_method == 'source' }}"
     venv_pip_packages: "{{ tempest_pip_packages + tempest_pip_plugins }}"
     venv_facts_when_changed:
       - section: "tempest"