Fedora images: use URLs from stable "archives.fedoraproject.org"

Fedora images: use URLs from stable "archives.fedoraproject.org"

The LinuxInitrd.test_with_2gib_file_should_work_with_linux_v4_16 test,
from tests/acceptance/linux_initrd.py, is currently failing to fetch
the "vmlinuz" file.  The reason for the failure is that the Fedora
project retires older versions from the "dl.fedoraproject.org" URL,
and keeps them in "archives.fedoraproject.org".  As an added note,
that test uses a Fedora 28 image, because of the specific Linux kernel
version requirements of the test.

For the sake of stability, let's use URLs from the archived and
supposedely ever stable URLs.  The good news is that the currently
supported versions are also hosted on the later.  This change limits
itself to change the URLs, while keeping the fetched files the same
(as can be evidenced by the unchanged hashes).

Documentation and the "vm tests" fedora definition were also updated.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
Reviewed-by: Yash Mankad <ymankad@redhat.com>
Message-Id: <20190904005218.12536-1-crosa@redhat.com>
Reviewed-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>