Added a default value for XQUEUE_RABBIT_HOSTNAME

Was causing a flood of error logs in /mnt/logs/edx.log

Oct 11 15:48:21 hostname [queue.management.commands.run_consumer][env:sandbox] INFO [hostname  1630] [run_consumer.py:69] -  [*] Starting queue consumers...
Oct 11 15:48:21 hostname [pika.callback][env:sandbox] DEBUG [hostname  1630] [callback.py:161] - Added: {'callback': <bound method Worker.on_connected of Worker (u'http://localhost:18050', u'edX-DemoX')>, 'only': None, 'one_shot': False, 'arguments': None}
Oct 11 15:48:21 hostname [pika.callback][env:sandbox] DEBUG [hostname  1630] [callback.py:161] - Added: {'callback': <bound method SelectConnection._on_connection_start of <pika.adapters.select_connection.SelectConnection object at 0x2383990>>, 'only': None, 'one_shot': True, 'arguments': None, 'calls': 1}
Oct 11 15:48:21 hostname [pika.connection][env:sandbox] DEBUG [hostname  1630] [connection.py:810] - Attempting connection
Oct 11 15:48:21 hostname [pika.adapters.base_connection][env:sandbox] DEBUG [hostname  1630] [base_connection.py:98] - Connecting the adapter to the remote host
Oct 11 15:48:21 hostname [pika.adapters.base_connection][env:sandbox] DEBUG [hostname  1630] [base_connection.py:153] - Creating the socket
Oct 11 15:48:21 hostname [pika.adapters.base_connection][env:sandbox] INFO [hostname  1630] [base_connection.py:163] - Connecting fd 5 to $XQUEUE_RABBIT_HOSTNAME:5672
Oct 11 15:48:21 hostname [pika.adapters.base_connection][env:sandbox] ERROR [hostname  1630] [base_connection.py:109] - socket error: Temporary failure in name resolution
Oct 11 15:48:21 hostname [pika.adapters.base_connection][env:sandbox] ERROR [hostname  1630] [base_connection.py:118] - Could not connect: Temporary failure in name resolution