Glance: Add 'db' to README.md
Glance: Add 'db' to README.md

The 'db' parameter is configurable so add it to the Glance README.md.

Change-Id: Ia3ed5e2d18d7cc0ba70b57c0da93a6dcb71e7cae

openstack/tripleo-image-elements,openstack/tripleo-image-elements,radez/tripleo-image-elements,rdo-management/tripleo-image-elements,rdo-management/tripleo-image-elements,radez/tripleo-image-elements