debconf/priority not respected

Starting with the bionic ISOs setting the debconf priority level via `debconf/priority=LEVEL` on the cmdline is not respected. Using `priority=LEVEL` however is.

Steps to reproduce:
1. Download daily bionic ISO from http://cdimage.ubuntu.com/ubuntu-server/daily/pending/

2. Pull out the kernel and initrd
$ mkdir iso
$ bsdtar xfp bionic-server-amd64.iso -C iso
$ cp iso/install/vmlinuz .
$ cp iso/install/initrd.gz .

3. Create QEMU disk
$ qemu-img create -f qcow2 vdisk.img 4G

4. Launch ISO via QEMU with debconf/priority set on cmdline:
$ qemu-system-x86_64 -enable-kvm -cpu host -m 1024 -boot d \
    -initrd initrd.gz -kernel kernel -display none -nographic \
    -hda vdisk.img -cdrom bionic-server-amd64.iso \
    -append 'console=ttyS0 locale=en_US.UTF-8 debconf/priority=critical'

The installer will ask you for keyboard layout, which is not expected as the priority on those questions is high, not critical.

5. Change 'debconf/priority=critical' to 'priority=critical' and the install will skip the initial keyboard and language questions, load additional modules, bring up the network, and finally ask you about a username and password, which is a critical question.

The expected behavior is that debconf/priority=critical and priority=critical are treated in the same manner.

Here are two example installer syslogs show how artful does and bionic does not respect the debconf/priority setting. In both you can clearly see it set on the cmdline, however see debconf does not respect it:

artful: http://paste.ubuntu.com/25970598/
bionic: http://paste.ubuntu.com/25970599/