Enable dracut deploy ramdisks

The element builds dracut from source on Ubuntu because the
Ubuntu dracut package is broken and very old, so it can't be
installed properly and causes a number of other issues that
are fixed by using a newer version of Dracut.

This initial version should work in virtualized environments.
Further validation of its suitability for real baremetal
deployments will need to be done in the future, but this should
be sufficient to enable that work.

Regarding Dracut specifically, in order to limit the changes
needed in the existing scripts this element continues to use a
cut down version of the /init script that we were building for the
existing ramdisk.  However, instead of running it as pid 0 it is
run as a Dracut pre-mount hook.  This allows Dracut to set up all
of the hardware and system bits, while falling early enough in the
Dracut sequence to complete the deployment before Dracut would try
to boot off the hard disk.

bp tripleo-juno-dracut-ramdisks
Change-Id: I144c8993fe040169f440bd4f7a428fdbe3d745cf