Add dist_upgrade Fabric action

Adds a dist_upgrade() function to run `apt-get dist-upgrade` in
non-interactive mode.

Command-line switch for apt-get are as follows:

- `--force-confdef` and `--force-confold` instruct `dpkg` to choose
  the default action when faced with installing a new version of a
  configuration file rather than keeping the old version. If there is no
  default, in which case `dpkg` would usually prompt for a choice,
  prefer to keep the old version of the file.
- `-y` assumes yes to prompts such as whether to proceed with the
  upgrades.
- `-q` reduces the output and omits progress indicators.
- `-u` will print the list of packages to be upgraded, useful for
  logging or later verification.
Prompt for confirmation before dist-upgrade

Prompt for confirmation before proceeding with `apt-get dist-upgrade`,
which can be a dangerous operation.

This will prompt until the user hits Ctrl-C, which is not the most
user-friendly but can be iterated on later.
Merge pull request #26 from alphagov/63458372-dist-upgrade

Add dist-upgrade task to apt.py