libvirt: Remove some unnecessary mocking in test_driver
libvirt: Remove some unnecessary mocking in test_driver

Calls to _fetch_instance_kernel_ramdisk already do nothing if kernel
and ramdisk are not defined. Their function is tested elsewhere.

_create_images_and_backing_files was being mocked where it wasn't
being used.

It's not necessary to mock configdrive.required_by when you can set
config_drive on the instance.

Change-Id: I6769ceefc10f075f2ae7629d9f7e8017974cb5b5

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