Update Image Guide for Newton
Update Image Guide for Newton

While updating the install guide, noticed two omissions in the
Image Guide:
- 'vhdx' disk_format was added in Newton with change
  I4e172c78d7afeb8be5a0123238efe3d8e4b044c9
- 'ova' container_format (was added a long time ago!)

Change-Id: I3bbc84023929da94c81806245987147b9757c43a

openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals