During my attempts to create a host, I hit the "submit" button a number of times. I would hit submit, see the error, then go try to fix it in other tabs (adding templates, fixing OS, etc.). When the host finally built, it could not find any boot media. It turns out the mac address in dhcpd.leases was wrong (perhaps from an earlier submit?). Creating a new host (with a different name) successfully worked and updated dhcpd.leases correctly.
Created from redmine issue http://projects.theforeman.org/issues/6622
Missing data on Redmine issue.