docker: Use bootloader --none to work around Anaconda regression

We were getting grub2 in the base image again.  Apparently
for a while Anaconda has supported a cleaner syntax for this, and
since it fixes the bug, let's use it.

https://bugzilla.redhat.com/show_bug.cgi?id=1222132

fedora-docker-base.ks

@@ -16,7 +16,7 @@
 #
 
 cmdline
-bootloader --location=none
+bootloader --disabled
 timezone --isUtc --nontp Etc/UTC
 rootpw --lock --iscrypted locked
 user --name=none