Add new modprobe-blacklist element.

This element can be used to provide a custom list
of modules via DIB_MODPROBE_BLACKLIST which
will be disabled via modprobe.d/blacklist.conf.

I'm using this to disable network adapters on the
Red Hat TripleO rack where the devices can't be
explicitly disabled via the BIOS.

Change-Id: I8a0a8ee05fa62628434d7f6422577dbf5cdd7a2e
Merge "Add new modprobe-blacklist element."