fixes #10298 - handle destroyed but present interfaces

When deleting a host and DHCP orchestration is removing reservations for NICs,
it generates a dhcp_record that contains next-server data. Determining the
next-server IP causes the primary interface to be fetched for DNS resolution,
to resolve the next-server hostname:

app/models/host/base.rb:390:in `get_interface_by_flag'
    app/models/host/base.rb:253:in `primary_interface'
    app/models/host/managed.rb:130:in `dns_ptr_record'
    app/models/host/managed.rb:838:in `to_ip_address'
    app/models/nic/managed.rb:16:in `to_ip_address'
    app/models/concerns/orchestration/dhcp.rb:49:in `boot_server'
    app/models/concerns/orchestration/dhcp.rb:72:in `dhcp_attrs'
    app/models/concerns/orchestration/dhcp.rb:18:in `dhcp_record'
    app/models/concerns/orchestration/dhcp.rb:32:in `del_dhcp'

get_interface_by_flag no longer attempts to modify the interface when it is
being deleted.