Add whole disk image creation command to Installation Guide
Add whole disk image creation command to Installation Guide

Installation Guide lacks whole disk user image creation guidelines.
To use agent drivers, we should use images created with the parameter
"vm", like: "disk-image-create ubuntu vm dhcp-all-interfaces -o my-image".

Change-Id: I82c30c20a1e1280beb8d30bfd3dd12df917172cb
Co-Authored-By: Lilia Sampaio <d9ba132a16dfdd5133795af67c0a94976b8c97a3@lsd.​ufcg.​edu.​br>
Closes-Bug: #1514955

openstack/ironic,ionutbalutoiu/ironic,openstack/ironic,NaohiroTamura/ironic,dims/ironic,dims/ironic,pshchelo/ironic,NaohiroTamura/ironic,bacaldwell/ironic,devananda/ironic,SauloAislan/ironic,pshchelo/ironic,bacaldwell/ironic,SauloAislan/ironic,hpproliant/ironic,ionutbalutoiu/ironic