Drop ancient discovery-related code

This code was in use when discoverd was a patch to Ironic and makes
no sense now. Stop putting discovery images in Glance, because
they're not consumed from Glance.
Switch to using upstream ironic-discoverd-ramdisk element

The element was backported from diskimage-builder master.
It mostly contains some clean ups.

We'll carry on the ramdisk development here, while trying to keep
the DIB version more or less up-to-date.
Merge pull request #92 from Divius/ironic-discoverd-1.0.0

Cleanups before switch to Kilo ironic-discoverd