Fix a typo in the driver.py file
Fix a typo in the driver.py file

there is a spelling error,"instace" should be "instance"

Change-Id: I2e7af7b1404e465f86ca3d963d3402640d888494

jianghuaw/nova,vmturbo/nova,sebrandon1/nova,Juniper/nova,vmturbo/nova,rajalokan/nova,jianghuaw/nova,hanlind/nova,klmitch/nova,gooddata/openstack-nova,openstack/nova,alaski/nova,mahak/nova,jianghuaw/nova,mikalstill/nova,alaski/nova,jianghuaw/nova,mahak/nova,hanlind/nova,gooddata/openstack-nova,vmturbo/nova,Juniper/nova,rajalokan/nova,klmitch/nova,mahak/nova,Juniper/nova,sebrandon1/nova,vmturbo/nova,gooddata/openstack-nova,openstack/nova,rajalokan/nova,mikalstill/nova,rahulunair/nova,sebrandon1/nova,hanlind/nova,mikalstill/nova,cloudbase/nova,Juniper/nova,rajalokan/nova,gooddata/openstack-nova,rahulunair/nova,rahulunair/nova,cloudbase/nova,cloudbase/nova,openstack/nova,phenoxim/nova,klmitch/nova,klmitch/nova,phenoxim/nova