use mirrorlist url instead of metalink url
There is an anaconda bug [1] where passing in metalink urls no longer
works after f25. Use mirrorlist urls to workaround.
[1] https://bugzilla.redhat.com/show_bug.cgi?id=1450129
(cherry picked from commit 77a25abded9538d5a0f625e3903d9be4d50d1af7)
Signed-off-by: Mohan Boddu <mboddu@redhat.com>