fix reading of MD redundcancy settings in grub/grub2

1. Ensure enable_md_array_redundancy is set when reading existing
configuration.

2. Show checkbox in wizard as enabled only when it is really enabled.

References: bnc#842919

Signed-off-by: Andrey Borzenkov <arvidjaar@gmail.com>
Merge pull request #25 from arvidjaar/fu/bnc/842919

fix reading of MD redundcancy settings in grub/grub2