[Functional] Prepare test logs archive only if logs are available

Recent change [1] moved all functional and fullstack tests to be
uploaded in single archive to Swift.
Sometimes, like e.g. in case of the networking-bgpvpn functional
tests, there is no logs directory at all so ansible tasks to prepare
tar.gz archive shouldn't be created at all.

[1] https://review.opendev.org/c/openstack/neutron/+/855868

Change-Id: I50cb1187ef772b6411fe91d6d74aba04a2d3e936

roles/prepare_functional_tests_logs/tasks/main.yaml

@@ -1,8 +1,14 @@
+- name: Check if {{ logs_path }} exists
+  stat:
+    path: "{{ logs_path }}"
+  register: logs_path_info
+
 - name: Prepare logs archive {{ log_archive_file_name }}
   become: yes
   shell:
     cmd: |
       /usr/bin/tar -czf {{ log_archive_file_name }} {{ logs_path }}
+  when: logs_path_info.stat.exists
 
 - name: Set {{ log_archive_file_name }} ownership
   become: yes
@@ -11,3 +17,4 @@
     owner: "{{ ansible_user }}"
     group: "{{ ansible_user }}"
     mode: 0775
+  when: logs_path_info.stat.exists