F19 GRUB configuration file

In the 51-grub hook, after the GRUB installation, the script will look
for a GRUB configuration file and in case it's not present one will
be generated using the grub-mkconfig command. The reason why it have
to be done is because the new Fedora 19 cloud images is using extlinux
by default.

Change-Id: I80b15b3122698d98ac4d47dc06faf5909a90ab00