Remove image_format config section

If the reference to the external-ceph-guide file configuration,
glance only supports raw image format.
I test can work normally of qcow2 image.
If enable ceph in kolla,the config section is not config.
see:
https://github.com/openstack/kolla-ansible/blob/master/ansible/roles/glance/templates/glance-api.conf.j2

Change-Id: Ie42be099692049b9f07e869e833b62000cff9dcc