Fix for Quantum LinuxBridge Intf driver plug call

The current implementation of the plug operation tries to assign an IP
address to a bridge even if it already has one. The fix is to do this
only when a new device is created. The check for a new device was
already being performed however the IP address was being set outside
that check. That's being corrected here, minor fix.

bug 1000580

Change-Id: I45215a3ca87a32ef4c2a2f76bab9b743eabb525e

nova/network/linux_net.py

@@ -1154,10 +1154,10 @@ def plug(self, network, mac_address, gateway=True):
             utils.execute('ip', 'link', 'set', bridge, 'up', run_as_root=True)
             LOG.debug(_("Done starting bridge %s"), bridge)
 
-        full_ip = '%s/%s' % (network['dhcp_server'],
-                             network['cidr'].rpartition('/')[2])
-        utils.execute('ip', 'address', 'add', full_ip, 'dev', bridge,
-                      run_as_root=True)
+            full_ip = '%s/%s' % (network['dhcp_server'],
+                                 network['cidr'].rpartition('/')[2])
+            utils.execute('ip', 'address', 'add', full_ip, 'dev', bridge,
+                          run_as_root=True)
 
         return dev