Add scripts/instack-prepare-discovery

Test as:
TFTP_ROOT=/tmp/tftp IRONIC_URL=http://127.0.0.1 ./instack-prepare-discovery

Places discovery.{kernel,ramdisk} and pxelinux.cfg/default into $TFTP_ROOT
Try to get Ironic URL from keystone for discovery configuration
Merge pull request #20 from Divius/setup-discovery-env

Add scripts/instack-prepare-discovery