Ubuntu system image 1.8

1.8 (2013-10-02)
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 * Support channel alias tracking. If the channel.ini file has a
   `channel_target` key, and the channel spec in the channel.json file has an
   `alias` key, and these don't match, then the channel alias has changed, and
   we squash the build number to 0 for upgrade path calculation. An explicit
   `--build` option for system-image-cli still overrides this. (LP: #1221844)
 * Support *phased updates* where we can ignore some images if their
   'phased-percentage' key is less than a machine-specific value.
   (LP: #1231628)
 * Switch the default `auto_download` value back to '1', i.e. download
   automatically but only over wifi. (LP: #1229807)
 * Plumb progress signals from ubuntu-download-manager through the
   system-image D-Bus API. (LP: #1204618)
 * Only send the `UpdateFailed` signal in response to a `CancelUpdate()` call
   if a download is already in progress. No signal is sent if there's no
   download in progress. Getting the files to determine whether an update is
   available or not does not count as a "download in progress". (LP: #1215946)