IPA as default ramdisk

This blueprint suggests to add support for pxe_* drivers (using the iSCSI methodology) to be able to use IPA as their deployment ramdisk.

The current bash ramdisk is hard to extend (bash scripting!), hard to debug (you can't ssh, there's no ssh server), and works in a very primitive way (there's no init system, the deploy script runs as PID 1). And IPA[1] is written in python, it's easier to debug (you can ssh into it), it's easy to extend (you can write stevedore extensions for it, just like we do for drivers in Ironic) and it's already running in production.

Also, having 2 deployment ramdisks is not nice, it requires features to be coded on both, it requires operators to generate and maintain different deploy ramdisks if he uses different drivers mixed (and if the nodes have different architetures it get's even more hairy).

So,for this work we need to:
 * add a new extension(s) to IPA do what the bash ramdisk was doing
 * extend the pxe_* drivers in Ironic to be able to deploy with both ramdisks (We can't just drop the bash deploy ramdisk, it should be backward compatible)
 * extend devstack
 * add a job in gate to test pxe_* drivers being deployed with IPA

[1] https://wiki.openstack.org/wiki/Ironic-python-agent