Update L3 based deployment spec
Update L3 based deployment spec

This change proposes simplification and usability improvements
to the previous design of this spec (see Change-Id
Id6314638cc949d7e2608385728529eb0f58e838d). Proposed changes
are largely enabled by the introduction of virtual media boot
capability to `redfish` hardware type.

Related-Bug: #1749193
Change-Id: I0ac7d4301b35f88d91f46bf6ddd47080b3148ee2
Story: 2006691
Task: 36986

openstack/ironic-specs