Valid IPv6 interface addresses must be accepted by the mgmtdomain API
endpoints. Additionally, an mgmtdomain can be either IPv4, IPv6 or both,
meaning that there are now multiple sets of minimally required
attributes to post to the endpoint.

This should ensure backwards compatibility with API clients that only
support ipv4_gw

The new version of this function can take the optional version argument
to select whether to get an address from the IPv4 or the IPv6 network of
the mgmtdomain.

This config setting will be used to choose which IP address version is
preferred as the primary management address of a device in a dual-stack
management domain.

This allows (but does not require) devices to be configured with
dual-stack management (even though CNaaS-NMS will continue to use the
primary management address).

This updates init_access_device_step1 to fetch, reserv and assign
secondary management IPs from any dual-stack Mgmtdomain.

Prefix length and mgmt_gw should be available variables also for the
secondary management address.

This repeats a lot of what `init_access_device_step1` does. Not sure
why there is an overlap, but I'm not going to change it without being
sure.

This also refactors redundant IP field value validation code. The logic
for all IP addr fields is the same, so no need for duplication.

This field is required for installations that want to run IPv6 directly
on the underlay.

Since this field is explicitly IPv6-only, it needs a separate check
from the other IPv6 fields.