Cinder-less BFV support

[Julia Kreger]

As part of some of the feedback received during the Sydney summit, some operators desired to be able to use the basic BFV functionality without cinder.

This would mean a noop storage interface that is geared towards stand-alone use that allows the drivers to transmit information to their HBAs or setup the PXE config to boot.

This has an additional consideration though, which is how do we get a bootable image out to a device if iPXE -> iSCSI chain loading is used. In a sense we might be able to adapt the current iscsi deploy process to perform the write-out, if it has sufficient access.

Etherpad discussion notes are available at: https://etherpad.openstack.org/p/ironic-queens-midcycle line 72

[Ruby Loo (rloo)]

Discussed this briefly with Julia [1]. Looks like we have some ideas, they ought to be flushed out, so I'd like a spec.

[1] http://eavesdrop.openstack.org/irclogs/%23openstack-ironic/%23openstack-ironic.2018-02-13.log.html#t2018-02-13T19:54:23