do not search for mirror named '<distro>-mirror' in dns by default

As described in the bug, enough non-cloud users experienced issues with
cloud-init selecting a mirror due to consumer level network providers using
dns server redirection.

We're turning this off by default.

cloudinit/config/cc_apt_update_upgrade.py

@@ -255,7 +255,7 @@ def find_apt_mirror(cloud, cfg):
             if mydom:
                 doms.append(".%s" % mydom)
 
-        if not mirror:
+        if not mirror and cfg.get(cfg, "apt_mirror_search_dns", False):
             doms.extend((".localdomain", "",))
 
             mirror_list = []

doc/examples/cloud-config.txt

@@ -28,11 +28,14 @@ apt_upgrade: true
 # then use the mirror provided by the DataSource found.
 # In EC2, that means using <region>.ec2.archive.ubuntu.com
 # 
-# if no mirror is provided by the DataSource, then search
-# for dns names '<distro>-mirror' in each of
+# if no mirror is provided by the DataSource, and 'apt_mirror_search_dns' is
+# true, then search for dns names '<distro>-mirror' in each of
 # - fqdn of this host per cloud metadata
 # - localdomain
 # - no domain (which would search domains listed in /etc/resolv.conf)
+# If there is a dns entry for <distro>-mirror, then it is assumed that there
+# is a distro mirror at http://<distro>-mirror.<domain>/<distro>
+#
 # That gives the cloud provider the opportunity to set mirrors of a distro
 # up and expose them only by creating dns entries.
 #
@@ -42,6 +45,8 @@ apt_mirror_search:
  - http://local-mirror.mydomain
  - http://archive.ubuntu.com
 
+apt_mirror_search_dns: False
+
 # apt_proxy (configure Acquire::HTTP::Proxy)
 apt_proxy: http://my.apt.proxy:3128