Fix roles variable name

In get-ansible-role-requirements playbook the variable
`roles` was changed to `required_roles`, but one variable
has been missing.

Change-Id: I2312dee57f004173257b24885c7e5c0bd6ddf085

tests/get-ansible-role-requirements.yml

@@ -71,7 +71,7 @@
                        selectattr('stat', 'defined') |
                        rejectattr('stat.exists') |
                        map(attribute='item') | list)
-                      | default(roles, True) }}"
+                      | default(required_roles, True) }}"
       register: git_clone
       retries: "{{ git_clone_retries }}"
       delay: "{{ git_clone_retry_delay }}"