Actually populate the file field

Thankfully, it turns out that HTTPBoot carries the same flexibility
as PXE with respect to no-ip answer being merged with an IP answer.

As a bonus, it actually is simpler than PXE, allowing skipping of udp
port 4011.  Of course, we still want to support PXE boot, so that will
need to be implemented.

confluent_server/confluent/discovery/protocols/pxe.py

@@ -378,6 +378,9 @@ def check_reply(node, info, packet, sock, cfg, reqview):
         bootfile = '{0}://{1}/confluent-public/os/{2}/boot/boot.img'.format(
             proto, info['netinfo']['recvip'], profile
         )
+        if not isintstance(bootfile, bytes):
+            bootfile = bootfile.encode('utf8')
+        repview[108:108 + len(bootfile)] = bootfile
     repview[20:24] = myipn
     gateway = None
     netmask = None