Description of problem:
When registering a host that has 3 NICs (eth0, eth1, eth2) where eth0 is IPv6-only and eth1,eth2 have neither IPv4 nor IPv6 addresses, Satellite selects an IP-less NIC (eth1 during my tests) as primary instead of selecting the only NIC that has an IP address (eth0 with an IPv6 address).

In a more schematic form:

== TEST HOST:
eth0: nice ULA-class IPv6 address; no IPv4 address.
eth1: no IP addresses.
eth2: no IP addresses.

Result: Satellite chooses eth1 as the primary interface.


== SECOND TEST:
eth0: nice ULA-class IPv6 address; no IPv4 address.
eth1: no IP addresses.
eth2: no IPv6 address; IPv4 manually added as 192.168.111.222/24. (note: Satellite does not know the 192.168.111.0/24 subnet.)

Result: Satellite chooses eth2 as the primary interface.



Note that this happens despite Satellite knowing the IPv6 subnet as expected:
* the IPv6 subnet used by the test host exists as a Subnet object on Satellite and is properly configured,
* Satellite itself has an IPv6 address on the same IPv6 subnet,
* Satellite can resolve the test host's name to its IPv6 address only, and can correctly reverse-resolve that IPv6 to the hostname.



Version-Release number of selected component (if applicable):
Satellite 6.9.7 (foreman-2.3.1.25-2.el7sat.noarch)
Satellite 6.10.1 (foreman-2.5.2.17-2.el7sat.noarch)

How reproducible:
100% of the times when using the Register Host method.


Steps to Reproduce:
1. Create IPv6 subnet on Satellite named subnet_ipv6, link it to a domain named example.ipv6.
2. Create on Satellite a hostgroup that links to the new IPv6 subnet and its respective domain and name it hg_ipv6.
3. Create host, give it no IPv4 address and an IPv6 address in the subnet_ipv6 subnet.
4. On the Satellite web UI, generate a Register script by navigating to Hosts > All Hosts > Register Host. Select the hg_ipv6 hostgroup and all other values appropriately.
5. Run the generated register command on the host.

Actual results:
Satellite will never select eth0 -- the NIC with only an IPv6 address -- as primary. In fact, Satellite will not record any IP addresses on eth0. Satellite will even choose eth1 (IP-less) over eth0. Satellite will even say eth1 is in subnet_ipv6 and is the interface whose FQDN matches the test host's name.

Alternatively, when eth1 or eth2 is given any IPv4 address -- even within a subnet Satellite does not know about -- Satellite will choose that NIC over eth0's perfectly functional IPv6 address.

Expected results:
Satellite would know to select the interface that actually works, even if it's an IPv6-only interface.

Additional info:
Output of some key Satellite and host configs will be uploaded to this BZ shortly.