Comment 2 for bug 719765

I had this error as well. The fix is:

sudo apt-get update
sudo apt-get install python-eventlet
sudo service nova-network start
ps -ef | grep nova-network
nova 3064 1 0 09:32 ? 00:00:00 su -c nova-network --flagfile=/etc/nova/nova.conf nova
nova 3071 3064 12 09:32 ? 00:00:00 /usr/bin/python /usr/bin/nova-network --flagfile=/etc/nova/nova.conf
root 3140 3563 0 09:33 pts/0 00:00:00 grep --color=auto nova-network