New hosts often fail due to not having an apt_mirror, because there
isn't one defined for their group and their host_vars haven't over-
ridden it.
We want new hosts to deploy successfully, so let's just use a default
apt_mirror if there isn't one defined. Rather have a slow mirror than
a failed deployment. And in any case, Linode can download from KENET's
mirror at 10MB/sec. ;)
Signed-off-by: Alan Orth <alan.orth@gmail.com>
