Add ability to use local cloud image

Adds the ability to set $DIB_LOCAL_IMAGE to use as the base cloud image
for a Fedora image build. There are many repetitve tasks that are done
every image build. With this change you can build an image with the
fedora and vm element and then reuse the resulting image as input to
future image builds. This greatly reduces future image build times.

For instance 99-up-to-date is already taking almost 2 minutes (even with
rpm downloads already cached) for Fedora 20, and is only going to keep
taking longer. By having a local up to date Fedora cloud image, this
time can be saved on each image build.

There is one minor change to support this in that /tmp/grub needs to get
cleaned up at the end of image builds so that the image can be reused.
Plus, there is no reason for it to stick around anyway.  (didn't think
this was worth a seperate commit).

Change-Id: Ic74d138da922ecc99c38c27f105170d90009a84a
Merge "Add ability to use local cloud image"