Rename PReP partitions to avoid confusion

In bnc#880049 we see a confusion what partition type we have to select
for GPT disk label. Rename it to make clear

Signed-off-by: Dinar Valeev <dvaleev@suse.com>
Adapt tests to libstorage PReP volume output

Signed-off-by: Dinar Valeev <dvaleev@suse.com>
Merge pull request #82 from k0da/master

Rename PReP partitions to avoid confusion