Install fedora grub from cached rpm during finalise

When uninstalling grub2, leave all its dependencies
including grub2-tools installed to minimise the number of packages
which need to be installed in the finalise stage.

Since the yum cache is unmounted during finalise, installing
grub2 in finalise is slowed by re-populating the yum cache.

This change copies the grub2 rpm out of the yum cache so it can be installed
from file during finalise.

This should prevent disk becoming full during finalise on Fedora.

Closes-Bug: #1217185

Change-Id: If095adc4abb52a19a3aa0b1caebfb3e4d8f605ef