Fix list rendering in bdm doc.
Fix list rendering in bdm doc.

What looked clear in the rst actually was far from clear when rendered
in HTML. The document was restructured a bit so all the options end up
in a single bullet list, and the combination description is a separate
section.

Part of bp: doc-migration

Change-Id: I2feee4018a332483658d24d299dbb04ec87f2df0

rahulunair/nova,mikalstill/nova,vmturbo/nova,jianghuaw/nova,mahak/nova,gooddata/openstack-nova,mahak/nova,vmturbo/nova,jianghuaw/nova,gooddata/openstack-nova,vmturbo/nova,klmitch/nova,jianghuaw/nova,klmitch/nova,vmturbo/nova,rahulunair/nova,jianghuaw/nova,phenoxim/nova,openstack/nova,klmitch/nova,phenoxim/nova,mahak/nova,gooddata/openstack-nova,klmitch/nova,openstack/nova,mikalstill/nova,mikalstill/nova,gooddata/openstack-nova,openstack/nova,rahulunair/nova