Fix vm-slug/prepare update-guestfs-applicance not found

The package update-guestfs-applicance could not be found when executing
vm-slug/prepare in ubuntu saucy: libguestfs-tools is required
options -o Dpkg::Options::='--force-confold' --force-yes were added to
apt-get install in order to disable the yes/no screen prompt.

Closes-Bug: #1295692
Change-Id: I4c83d650509cc06f066d7183ac9be7e73de44283
Merge "Fix vm-slug/prepare update-guestfs-applicance not found"