Looking at the subnet setting it's likely caused by the fact the interface is attached to a subnet which IPAM is set to Internal DB. We need few tests but IIRC the UI does the autosuggestion so it works there. A temporary workaround is to set the IP manually as another argument for that NIC.

Created redmine issue http://projects.theforeman.org/issues/18817 from this bug

Build : Satellite 6.2.15 snap1

hammer -d host create --hostgroup-id 2 --location "Default Location" --organization "Default Organization" --name robtest-sjagtap --compute-resource libvirt --compute-profile 2-Medium --lifecycle-environment DEV --content-view "RHEL 7 CV" --interface identifier=eth0,primary=false,provision=true,managed=true,subnet_id=2,compute_type=bridge,compute_network="testingsat6_el7",compute_model=virtio --interface identifier=eth1,primary=true,provision=false,managed=true,subnet_id=3,compute_type=bridge,compute_network="testingsat6_el7",compute_model=virtio

I was able to create a host and IP address was assigned to both the interfaces

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://access.redhat.com/errata/RHBA-2018:1672