Description of problem:
I'm unable to install Fedora on precreated software RAID1 partitions.

Steps to Reproduce:
1. Create software RAID partitions before starting Fedora installation.
2. Run installer.
  
I created software RAID1 partitions in Ubuntu, then run Fedora installation. At first it didn't even recognize those partitions. I found out I need to mark those partitions as 0xfd to make them show up in anaconda, but then another problem arised.
When partitions are marked as 0xfd(linux software RAID autodetection) the kernel sets them up as /dev/md_d[0-n], but anaconda is trying to use /dev/md[0-n] names.
I tried disassembling kernel created md devices, then selected custom layout in anaconda, and chose md partition as root. After confirming the layout anaconda rereads partition tables (even though partition layout didn't changed) which causes kernel to recreate and assemble raid devices under /dev/md_d[0-n] names.
In the same time anaconda is trying to assemble the same raid devices under /dev/md[0-n] names.

Example:
my RAID partitions:
/dev/md0 RAID1 (/dev/sda1, /dev/sdb1)
/dev/md1 RAID1 (/dev/sda2, /dev/sdb2)
/dev/md2 RAID1 (/dev/sda5, /dev/sdb5)
/dev/md3 RAID1 (/dev/sda6, /dev/sdb6)

After confirming the layout it looks like this:
/dev/md0 RAID1 (/dev/sda1, missing)
/dev/md1 RAID1 (/dev/sda2, missing)
/dev/md2 RAID1 (/dev/sda5, missing)
/dev/md3 RAID1 (/dev/sda6, missing)

/dev/md_d0 RAID1 (missing, /dev/sdb1)
/dev/md_d1 RAID1 (missing, /dev/sdb2)
/dev/md_d2 RAID1 (missing, /dev/sdb5)
/dev/md_d3 RAID1 (missing, /dev/sdb6)

So anaconda assembled RAID partitions are degraded and installation is aborted.

I tried adding raid=noautodetect to kernel boot up options, but kernel still created /dev/md_d[0-n] devices automatically. And when I mark partitions that are part of RAID devices as i.e. 0x83 or 0x82, they won't even show up in anaconda during custom partition layout creation. So in the end I'm unable to install Fedora on software RAID devices.