Python string.find method returns -1 if it can't find the substring (https://docs.python.org/2/library/string.html, see string.find). So, you could modify your yml like that:
- name: set /etc/timezone
shell: echo "{{ timezone }}" > /etc/timezone
when: get_timezone.stdout.find('{{ timezone }}') == -1
notify: update tzdata
or just use "not in":
- name: set /etc/timezone
shell: echo "{{ timezone }}" > /etc/timezone
when: '"{{ timezone }}" not in get_timezone.stdout'
notify: update tzdata