Fix for unexpected entries in /sys/class/net

For example, in a bonding configuration there is a file there, not just
directories with ifindex entries.

confluent_server/confluent/util.py

@@ -31,6 +31,8 @@ def list_interface_indexes():
     # For now be linux specific
     try:
         for iface in os.listdir('/sys/class/net/'):
+            if not os.path.exists('/sys/class/net/{0}/ifindex'.format(iface)):
+                continue
             ifile = open('/sys/class/net/{0}/ifindex'.format(iface), 'r')
             intidx = int(ifile.read())
             ifile.close()