Also adds a grubcfg.conf with the following settings:
* overwrite: If set to true, always creates /etc/default/grub from
scratch even if the file already existed. If set to false, edits the
existing file instead. The default is "false".
* defaults: Default entries to write to /etc/default/grub if it does not
exist yet or if we are overwriting it. Note that in addition,
GRUB_CMDLINE_LINUX_DEFAULT and GRUB_DISTRIBUTOR will always be
written, with automatically detected values. The default in the code
is empty. The shipped grubcfg.conf currently reproduces the default
settings from the Fedora installer Anaconda.
Fixes#128.
