Move utility functions out of mininet.py

Having a file with the same name as its package creates hard-to-diagnose
import error, so rename mininet.py to net.py.  This commit moves utility
functions, generally those dealing with network namespaces and interface
management, into their own file.

Also fix a few minor formatting bugs and clean up logging code (which
had issues when multiple files would import mininet.logging_mod).

mininet/test/test_nets.py

@@ -7,7 +7,7 @@ Test creation and all-pairs ping for each included mininet topo type.
 from time import sleep
 import unittest
 
-from mininet.mininet import init, TreeNet, LinearNet, pingTest, DATAPATHS
+from mininet.net import init, TreeNet, LinearNet, pingTest, DATAPATHS
 
 class testMinimal(unittest.TestCase):
     '''For each datapath type, test ping with a minimal topology.