Instead of handling iBFT (iSCSI Boot Firmware Table) in the ifcfg-rh plugin,
create a new plugin for it.  This allows all distributions to use iBFT
configuration, and makes both iBFT handling and ifcfg-rh less complicated.

The plugin (like the old ifcfg-rh code) creates read-only connections backed
by the data exported by iscsiadm.  The plugin does not support adding new
connections or modifying existing connections (since the iBFT data is
read-only anyway).  Instead, users should change their iBFT data through
the normal firmware interfaces.

Unmanaged devices can be configured through NetworkManager.conf and the
normal 'keyfile' mechanisms.

(In the future, we'll read this data directly from the kernel's
/sys/firmware/ibft/ethernetX directory instead of iscsiadm, since the
kernel has all the information we need and that's where iscsiadm gets
it from anyway.)

https://bugzilla.gnome.org/show_bug.cgi?id=734009