Add apt_preferences file with bootcmd

The apt_preferences file must be written
before we run "apt-get upgrade", or the
pinning does not take effect on non-bootstrap
machines.

Fixes https://bugs.launchpad.net/juju-core/+bug/1370781
Merge pull request #813 from axw/lp1370781-upgradejuju-pinned-1.20

Add apt_preferences file with bootcmd

The apt_preferences file must be written
before we run "apt-get upgrade", or the
pinning does not take effect on non-bootstrap
machines.

Fixes https://bugs.launchpad.net/juju-core/+bug/1370781