Allow hostname lookup for flexible --serverInstance values

[stephen-soltesz]

When running the replay_client.py and specifying a serverInstance that is not in the self.ips map, the client aborts.

This change adds a try/except block around the socket.gethostbyname to first try self.ips and then uses the machineName directly.

$ python src/replay_client.py --pcap_folder=./replayTraces/Vimeo_12122018/ --serverInstance=wehe-mlab1-lga0t.mlab-sandbox.measurement-lab.org
 [1]Reading configs file and args)
Traceback (most recent call last):
  File "src/replay_client.py", line 1082, in <module>
    main()
  File "src/replay_client.py", line 1079, in main
    run()
  File "src/replay_client.py", line 862, in run
    initialSetup()
  File "src/replay_client.py", line 1049, in initialSetup
    configs.set('serverInstanceIP', Instance().getIP(configs.get('serverInstance')))
  File "/Users/soltesz/src/github.com/m-lab/wehe_desktop/src/python_lib.py", line 565, in getIP
    ip = socket.gethostbyname( self.ips[machineName] )
KeyError: 'wehe-mlab1-lga0t.mlab-sandbox.measurement-lab.org'

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This change is