Set default tempest_install_method for meta handlers

For non-integrated tests tempest_install_method is not defined,
so when python_venv_build tries to launch meta handler, it fails with
undefined variable. Once default is set, this will prevent problems

Depends-On: https://review.opendev.org/#/c/665396/

Change-Id: I3cdfe48cae28969dea381bfa325c4bb4769fb4d1

tasks/tempest_install.yml

@@ -25,9 +25,13 @@
   retries: 5
   delay: 2
 
+
+# NOTE(noonedeadpunk):
+# Applying default filter for tempest_install_method is required despite the variable being defined
+# in defaults as in case of non-integrated tests meta handlers from other roles might fail.
 - name: Install Tempest packages from PIP
   import_tasks: tempest_install_source.yml
-  when: tempest_install_method == 'source'
+  when: (tempest_install_method | default('source')) == 'source'
 
 - name: Install stackviz on venv
   import_role: