grubcfg: Create /etc/default/grub if missing.

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.