backports_url was already set to the same value than debian_url by
default, but it was set to the default value of debian_url. this meant
that when giving a url to the debian_url parameter to the apt class, the
backports_url would not follow and would still be using the default
value for debian_url.
with this change, when backports_url is not specified, but debian_url
is, then both of them take on the value given to the debian_url. of
course if backports_url is also given a value, then it keeps the value
given by the user.
770f8fb5da