Remove serialized collect of ansible_default_ipv4

The fallback_ips tasks are essentially serializing the gathering of one
fact on all the hosts, which can have dramatic performance implications
on large clusters (several minutes).

This is essentially a reversal of 35f248df
Being able to run without refreshing the cache facts is not worth it.

We keep fallback_ip for now, simply changing the access to a normal
hostvars variable instead of a custom dictionnary.