Currently if a cloud config sets force_ipv4, it will still have the
public_v6 field populated if the provider API gives an IPv6 address.
I can not see why you would want this address populated if you are
telling the cloud to force IPv4. As a concrete example;
If6e1a0402b9b7f93cc76623c01049764abc68b2a proposes in zuul-jobs adding
the IPv6 address to /etc/hosts for multinode jobs. It does this by
walking the nodepool interface values, which on some clouds with
force_ipv4 set will have invalid/unconfigured IPv6 entries.
I've updated the documentation to expand a bit more on what situations
this flag might be useful, which AIUI is really mostly about clouds
that return you an IPv6 address in the API but don't give you a
practical way to auto-configure it.
Change-Id: I7aaaf44ab1a1d4d25225843227ef6ab6d8564063
661a0eb4b5