On Fedora 31 passworless root login is no longer working. We already
install a ssh key, may as well use it.
This also reduces the live boot timeout to 2s from 60s, which should
help with timeout problems when booting.
Cherry-picked from e3654e8297
Related: rhbz#1769525
This splits out the lorax-composer specific execution so that the built
image can be downloaded from the build vm and booted by the host instead
of using nested virt to try and boot it inside the build vm.
Also adds copying the ssh key from the build vm so that it can log into
the image and run the test_boot_* scripts.
Cherry-picked from df7a018ee2
Related: rhbz#1769525
