traffic_replay: Avoid Exception if no packet rate is specified

traffic_replay: Avoid Exception if no packet rate is specified

traffic_replay would throw an exception if you didn't specify some sort
of packet rate. We can avoid this by using --scale-traffic=1.0 as the
default if nothing else was specified.

 script/traffic_replay model.txt $SERVER.$REALM --duration=10
   --fixed-password=blahblah12# -U$USERNAME%$PASSWORD
INFO 2019-04-10 01:03:01,809 pid:47755 script/traffic_replay #280: Using
the specified model file to generate conversations
Traceback (most recent call last):
  File "script/traffic_replay", line 438, in <module>
    main()
  File "script/traffic_replay", line 293, in main
    opts.conversation_persistence)
  File "bin/python/samba/emulate/traffic.py", line 1295, in
generate_conversation_sequences
    target_packets = int(packet_rate * duration)
TypeError: unsupported operand type(s) for *: 'NoneType' and 'float'

Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>