mockremote: rawhide instead of $releasever in repos when in rawhide chroot

When we add path to repo in repolist we adding
http://xxx/fedora-$releasever-$basearch. Mock should interp this as
http://xxx/fedora-rawhide-$basearch for rawhide chroots.
We will replace "$releasever" with "rawhide" when we building in rawhide
chroot.
We could request from user release repos and rawhide repos, but this isn't convenient.

Reference: https://bugzilla.redhat.com/show_bug.cgi?id=1055325
Signed-off-by: Igor Gnatenko <i.gnatenko.brain@gmail.com>