Remove old versions of grub2 from the yum cache

Before downloading and caching the latest version of grub2, we need to
first remove all old versions. Otherwise, the find command that writes
the grub2 version to /tmp/grub/install to install during the finalise
phase could pick one of the older versions of grub2 in the cache. This
will cause the install during the finalise phase to fail since the older
version of grub2 will conflict with the newer installed version of
grub2-tools.

Change-Id: I5b43e359db3ba81ed6283dfb41c7e89629516d38
Merge "Remove old versions of grub2 from the yum cache"