Install libguestfs for nova-compute on Ubuntu

We were already installing this for n-cpu on rpm distros, but not
Ubuntu.  Install it so that nova-compute can use it for file injection,
which is the preferred method over nbd.

Set CONF.libvirt.inject_partition to -1.  This enables using libguestfs to
determine the proper partition to inject into.

Don't bother trying to load the nbd kernel module anymore.  It won't be used
since we know always expect libguestfs to be installed.

Change-Id: Ifa9d95bf759f1dad8685590a2df242d852dd2cb0
Merge "Install libguestfs for nova-compute on Ubuntu"