Add some more comments to _get_my_ip().

bug 930513.

This patch adds some additional comments to _get_my_ip() to try to make
the code a bit more clear and to clarify that no traffic is actually
sent out by this code.

Change-Id: I6f8d4a0a51596e5c531da53f3c79c5bffca59b39

nova/flags.py

@@ -59,7 +59,14 @@ def DECLARE(name, module_string, flag_values=FLAGS):
 
 
 def _get_my_ip():
-    """Returns the actual ip of the local machine."""
+    """
+    Returns the actual ip of the local machine.
+
+    This code figures out what source address would be used if some traffic
+    were to be sent out to some well known address on the Internet. In this
+    case, a Google DNS server is used, but the specific address does not
+    matter much.  No traffic is actually sent.
+    """
     try:
         csock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
         csock.connect(('8.8.8.8', 80))