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.
Add sysctl to all images

This is needed to customize the default kernel keepalive timings, see
https://bugzilla.redhat.com/show_bug.cgi?id=1167414
Merge pull request #94 from gfidente/sysctl

Add sysctl to all images
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
Add Tuskar UI Extras