Instance network boot (not to be confused with ramdisk, iSCSI or
anaconda deploy methods) is insecure, underused and difficult to
maintain. This change removes a lot of related code from Ironic.
The so called "netboot fallback" is still supported for legacy boot when
boot device management is not available or is unreliable.
Change-Id: Ia8510e4acac6dec0a1e4f5cb0e07008548a00c52
f0a1778766