Remove image requirement for boot from volume
Remove image requirement for boot from volume

Section previously implied an image had to be specified
when booting from volume. This was true previously but
has since been corrected and thus it is not necessary
to specify an image to successfully boot from a volume.

Change-Id: I85d61726f00ece33e453ad739c4955b5f8082353
Closes-Bug: #1269767

gongysh2004/openstack-manual,AlekhyaMallina-Vedams/openstack-manuals,openstack/openstack-manuals,Akasurde/openstack-manuals,openstack/openstack-manuals,potsmaster/openstack-manuals,kairoaraujo/openstack-manuals,saeki-masaki/openstack-manuals,gongysh2004/openstack-manual,openstack/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,potsmaster/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,Akasurde/openstack-manuals,openstack/openstack-manuals,kairoaraujo/openstack-manuals,saeki-masaki/openstack-manuals