Start using generic dracut-regenerate element
Start using generic dracut-regenerate element

In latest versions of diskimage-builder, a generic dracut-regenerate
element had been added. So we could start using this one instead
of the custom tripleo one.

Change-Id: Iecfc926c9d9a169703ed1df7e2980ec440fb216d

openstack/tripleo-common,openstack/tripleo-common