Fix condition evaluation on ansible_all_ipv6_addresses to have a Boolean result (#404)

The condition `when: ansible_all_ipv6_addresses` evaluates to a list, which in
term evaluates to True,  but it seems that more recent Ansible versions do not
accept that anymore:

```
Conditional result was "['fe80::...']" of type 'list', which evaluates to True. Conditionals must have a boolean result.
```

This commit adds a length check to create a real Boolean result.

Signed-off-by: Stefan Haun <mail@tuxathome.de>